Soil secondary salinization an escalating problem worldwide, especially in arid and semi-arid regions. Plants growing in salt-affected soils may suffer from physiological drought stress, ion toxicity, and mineral deficiency that lead to reduced growth and biomass. At present, in the study of secondary salinization soil amelioration, more attention is focused on the influence of soil salt ions, and soil physical and chemical properties than on the evaluation of amelioration and vegetation restoration. However, this assessment does not simultaneously consider the effects on the growth and physiological conditions of vegetation. Utilization of the straw layer in soil is a widely used and effective measure for soil salinity control. To study the response of the spectral reflectance and photosynthetic characteristics of fast-growing poplar (Populus) to different straw interlayers, a field experiment was conducted in the typical secondary salinized land of the northwestern salinized and ecologically fragile area of Ningxia. The experiment was established with four treatments: straw interlayer (B), straw mulching (M), straw interlayer plus straw mulching (B+M), and no addition of straw interlayer (CK). All treatments were arranged in a randomized complete block design with four blocks. Based on the change of soil salt and water in the background, the response of the straw layer was studied by analyzing the spectrum and photosynthetic characteristics of the fast-growing poplar. The photosynthetic rate, pigment content, nutritional status, and stress of the above-ground plants under different straw layer conditions were measured to more accurately reflect the effect of each treatment on saline soil improvement and above-ground vegetation restoration. It is of great significance to further study the physiological indexes, such as reflectance spectra and photosynthetic characteristics of plants. The results showed that: (1) different straw layers had different effects on water and salt regulation, resulting in different soil conditions among different treatments, which caused changes in pigment content, nutritional status, and physiological and biochemical conditions of the plants; (2) by detecting the plant reflection spectrum and photosynthetic characteristics, changes in plant growth and physiological conditions could be reflected. Based on this, we could use the reflectance spectrum and photosynthetic characteristics of fast-growing poplar to respond to different straw layers and more accurately assess improvement measures. The maximum net photosynthetic rate and saturation light intensity of B, M, and B+M treatment were significantly increased, and the B and B+M treatment light compensation points and dark respiration rates were significantly reduced. B and B+M treatment significantly increased the leaf photosynthetic rate (Pn), transpiration rate (Tr), stomata conductance (Gs), and intercellular CO₂ concentration (Ci). The spectral reflectance of plant leaves, chlorophyll content of the B+M treatment was highest, and photosynthetic and nutrition performed best. Compared with that of the control group, other spectral reflectance indices significantly improved. The B and M treatments also significantly improved chlorophyll content and photosynthetic characteristics, but were less effective than B+M; (3) the experimental study showed that the reflectance spectrum and photosynthetic characteristics of fast-growing poplar responded differently to different straw layers. The straw layer helped to improve the physiological condition of fast-growing poplar. Based on the results of comprehensive reflection spectrum and photosynthetic characteristics, B+M was the most suitable straw layer model for the improvement of secondary saline-alkaline land in the Northwest Yellow River irrigation area.