Pinus densiflora has grown in the southern Horqin Sandy Land, China, for 50 years, but little is known about its field performance in this region, and nothing is known about its photosynthetic physiology. To understand the physiological characteristics and mechanisms of photosynthesis in P. densiflora, mature trees were compared with the same-aged trees of Pinus sylvestris var. mongolica (the control), and the growth characteristics of both species were investigated. Photosynthetic characteristics were analyzed with a Li-6400 system, and fast chlorophyll a fluorescence transients (OJIP) were analyzed with a Pocket PEA plant efficiency analyzer. Growth and biomass of mature P. densiflora was greater than that of P. sylvestris var. mongolica. This was closely related to stronger photosynthesis in P. densiflora, including a higher maximum net photosynthetic rate (P _max, 10.376 μmol CO₂ m^-2 s^-1) and a large range of light adaptation with high light saturation point and low light compensation point. The dark respiration rate (R_d) of P. densiflora was low, leading to higher photosynthetic efficiency (P_max/R_d). Diurnal changes in the net photosynthetic rate (P_n) of P. densiflora ranged from 3.290 to 7.349 μmol CO₂ m^-2 s^-1, and the mean P_n was 4.902 μmol CO₂ m^-2 s^-1, which was 36.2% higher than that of P. sylvestris var. mongolica. The P_n of P. densiflora at each time point was significantly higher than that of P. sylvestris var. mongolica, except at 14:00 and 16:00. The transpiration rate of P. densiflora was 34.8% lower than that of P. sylvestris var. mongolica, and its water use efficiency was 2.06-fold greater than that of P. sylvestris var. mongolica. The P_n of P. densiflora remained high when stomatal conductance was low. Compared with P. sylvestris var. mongolica, P. densiflora was less susceptible to dynamic photoinhibition, showing slight photoinhibition only at a PPFD of 2000 μmol m^-2 s^-1, when P_n was 91.4% of the maximum. The chlorophyll content in P. densiflora needles was 1.380 mg/g, double that in needles of P. sylvestris var. mongolica. The JIP test was used to reveal the mechanism for the strong photosynthesis of P. densiflora and showed that in the range from O to P phase, the relative variable fluorescence value of P. densiflora was generally low. This low value indicated that little energy was dissipated via the electron transfer chain, so that more energy could be used for photochemistry. The two species reached fluorescence maxima at different times under saturating illumination: 700 ms for P. densiflora and 900 ms for P. sylvestris var. mongolica. The phenomenological energy fluxes per excited cross section (CS) were calculated; these indexes showed that, compared with P. sylvestris var. mongolica, P. densiflora had a higher density of reaction centers (RC/CSo, RC/CSm), a greater proportion of photon flux absorbed by the antenna pigments (ABS/CSo, ABS/CSm), higher trapping flux to the reaction center (TRo/CSo), and used more energy for electron transfer (ETo/CSo). These attributes led to its higher performance indexes. The PI_{(ABS/CSo/CSm)} of P. densiflora were 1.42-, 1.65-, and 1.63-fold those of P. sylvestris var. mongolica, respectively. Together, these results suggest that the growth and photosynthetic performance of P. densiflora are better than those of P. sylvestris var. mongolica in the Horqin Sandy Land environment. This conclusion provides the theoretical basis for the expansion of P. densiflora cultivation in this region.