Plant photosynthetic capability usually changes after damage by a herbivorous pest. Photosynthetic compensation is the physiological response of the plant to pest damage with the level of compensation varying with the change in pest damage. This paper explores the photosynthetic response mechanism of alfalfa to the dominant insect-Odontothrips loti -and explains the compensatory mechanism of alfalfa to thrip damage. The thrip resistant clone, R-1 and susceptible clone, I-1 were used to investigate the gas exchange and chlorophyll fluorescence parameter changes under different insect densities (0, 1, 3, 5, and 7 per branch, respectively), Photosynthesis equipment, GFS-3000 (Walz,Germany) and modulated chlorophyll fluorometer imaging-PAM (Walz,Germany) were used. For the 7 per branch treatment, the results indicate that the chlorophyll content of R-1 initially increased and then decreased, while the chlorophyll content of I-1 decreased. For R-1, the chlorophyll content was 11.32% lower than CK (0 thrip per branch), and for the 3, 5, and 7 per branch treatments of I-1, the chlorophyll contents were 14.05%, 22.02% and 26.27% lower than CK, respectively. For both R-1 and I-1, the net photosynthetic rate (P_n) and water use efficiency (WUE) decreased, while the intracellular concentration of CO₂ (C_i), stomatal conductance (G_s) and transpiration rate (T_r) increased. For R-1, the P_n of 3, 5, and 7 per branch treatments were 6.98%, 19.03% and 20.11% lower than CK, and the WUE of all the treatments were 16.32%, 23.95%, 37.12% and 45.89% lower than CK. For I-1 treatments, the P_n of all the treatments were 5.38%, 8.77%, 22.47% and 35.66% lower than CK, and the WUE were 25.23%, 31.05%, 45.78% and 61.81% lower than CK, respectively. The chlorophyll content, WUE and P_n of R-1 were all greater than I-1 under the same insect density. As insect density increased, the initial fluorescence (F₀) increased, which for the R-1 clone resulted in F₀ for 5 and 7 per branch treatments of 6.99% and 9.13% higher than CK, respectively. For the I-1 clone, all the treatments were 2.81%, 6.45%, 12.36% and 14.93% higher than CK, respectively. The actual photosynthetic efficiency (Ф_PSⅡ) of PSⅡ, non-photochemical quenching coefficient (NPQ), photochemical quenching coefficient (qP), potential activity (F_v/F₀) of PSⅡ and original light transformation efficiency (Ф_PSⅡ) of PSⅡ decreased for both R-1 and I-1. Among which the F_v/F₀ of 3, 5 and 7 per branch treatments for the R-1 clone were 5.07%, 16.74% and 21.19% lower than CK and the F_v/F_m of 5 and 7 per branch treatments were 3.50% and 4.63% lower than CK, respectively. For the I-1 clone, F_v/F₀ of all the treatments were 8.24%, 13.68%, 22.88% and 28.04% lower than CK, and the F_v/F_m were 1.67%, 2.91%, 5.31% and 6.86% lower than CK, respectively. Under the same insect density, R-1 was found to have a lower F₀ but higher Ф_PSⅡ, qP, F_v/F₀ and F_v/F_m, when compared with I-1. As a rule, the gas exchange parameter and chlorophyll fluorescence kinetic parameter of R-1 fluctuated less than I-1, indicating that the thrip's rasping-sucking damage had injured the chloroplast tissue of alfalfa leaves, decreased the anabolism of chlorophyll, aggravated leaf transpiration, decreased WUE, and therefore affected alfalfa photosynthesis. The thylakoid membrane in the alfalfa leaves and PSⅡ reaction center were injured, which decreased the absorption of light energy, and impeded the photosynthetic electron transport, reducing its photosynthetic efficiency. While under lower insect densities (1 per branch, 3 per branch), the R-1 clone had a stronger capability to adjust for water loss and usage after being damaged by the thrip, demonstrating adaptability to the thrip's rasping-sucking damage through internal regulation, lowering PSⅡ damage, with higher absorption, transmission, use and conversion efficiency. Therefore the R-1 clone was found to have a stronger resistance to thrips when compared with the I-1 clone, as expressed by the higher photosynthetic efficiency and photosynthetic compensation effect.