In their evolution, grasses have developed symbiotic associations with fungi including mycorrhizal fungi that grow in or on the roots, and endophytes that live their entire life cycle within the aerial portion of the host grass, typically forming nonpathogenic, systemic and usually intercellular associations. Currently, seven genera have been identified, including Atkinsonella, Balansia, Balansiopsis, Echinodothis, Epichloё, Myriogenospora and Parepichloё. Among them, the anamorphic (imperfect) stage of Epichloё spp., i.e. Neotyphodium spp., is most closely related to cultivated grasses. The most widely known Neotyphodium endophytes are N. lolii and N. coenophialum, which colonize perennial ryegrass (Lolium perenne L.) and tall fescue (Festuca arundinaceae Schreb.) respectively. Endophytes may protect the hosts from biotic and abiotic stresses. In the past twenty years, much research has focused on the beneficial effect of endophyte infection on the biotic stress resistance of host grasses such as mammalian, insect and nematode herbivores. When abiotic stress was regarded, many studies showed that endophyte infection could improve drought resistance ability of host grasses. Similar to drought stress, salt stress also led plants to physiological drought. But up to now, no related research results were reported. Thus, in this paper Lolium perenne cv SR4000 infected by Neotyphodium lolii was chosen as experimental material. Vegetative growth and several physiological indexes (such as free proline content, soluble sugar content and photochemical efficiency of PSII) of endophyte-infected (EI) and endophyte-free (EF) populations under normal and salt stress conditions were compared in order to discuss the effect of endophyte infection on the salt resistant ability of the host ryegrass. The experiment was performed under greenhouse condition. EI and EF plants were transplanted into culture pots filled with 1L 1/2 Hoagland solution. One week later, salt (NaCl) was added to impose salt stress (control, low and high stress). Salt content for the above stresses was 0, 0.3% and 0.6%, respectively. Each treatment was replicated 5 times. From the beginning, tiller number and free proline content of each population were recorded every 5 days. Photochemical efficiency of PSII of experimental populations was surveyed at the middle and late period separately. At the end of the experiment, shoots and roots were harvested and weighed separately. At the same time, soluble sugar content of both shoots and roots was measured. The whole experiment lasted 35 days. During the period, distilled water was supplied whenever the solution level in the pot dropped due to transpiration and evaporation. The results showed that endophyte infection did not improve shoot growth of ryegrass. Under high salt stress, tillering ability and shoot biomass of EI populations were all lower than those of EF populations. However, endophyte infection was beneficial to roots of ryegrass, which was demonstrated by the fact that root biomass of EI populations, under salt stresses, decreased more slowly than that of EF populations. When it came to the ratio of root to shoot, no significant change was observed between under control and under high salt stress for EI populations, while for EF populations, it decreased significantly in comparison with control, and the decrease rate was up to 41.7%. Taking these factors in consideration, under high salt stress, EI populations may maintain root growth at the cost of shoot growth, which may be beneficial to their survival. Salt stresses led to increase of free proline content of ryegrass leaves, and the degree of increase was heightened with extended stress time and advanced stress level. When compared with EF populations, proline content of EI populations was higher and especially significant under high stress. As for the content of soluble sugar, its value was similar under both control and low salt stress in EI and EF populations. Under high salt stress, however, the sugar content of EI populations was significantly lower than that of EF populations, which was considered to be unbeneficial to host grass’ survival. Moreover, during the experimental period, photochemical efficiency of PSⅡ was measured three times, but no significant difference was observed between EI and EF populations. In conclusion, endophyte infection did not alleviate negative effect of salt stress on the host ryegrass.