Abstract: Objective A field experiment was conducted to study the effects of fertilizer dosage on the biomass, photosynthetic characteristics as well as the fast chlorophyll fluorescence on walnut in mountainous region in order to select suitable fertilizer dosage and provide theoretical basis for high yield and good quality of walnut in Northwest mountainous region.Method Using ten-year-old walnut variety "Lu Guang" as materials, random sampling group design method was used in field experiments. The photosynthetic characteristics of walnut were analyzed by measuring the growth amount, photosynthetic diurnal variation, response curve and fast chlorophyll fluorescence. Result The daily variation of net photosynthetic rate of walnut in mountainous region presented a bimodal curve, within a certain range, the biomass, chlorophyll content, photosynthetic parameters increased with increasing fertilizer dosage. But high fertilizer dosage treatment lead to the chlorophyll content and photosynthetic rate decreased and lower than that of CK, especially under high photosynthetic active radiation. The maximum net photosynthetic rate (Pnmax2) of the walnut leaves under saturated intercellular CO2 concentration was higher than that under saturated irradiance, indicating that the photosynthetic efficiency was limited to the great extent by CO2 supply. The chlorophyll fluorescence OJIP curve of walnut was deformation to OKJIP curve under high temperature and strong light at noon. The K and J points of the fast chlorophyll fluorescence induction dynamics curve under high fertilizer dosage treatments were higher than other treatments, indicating that high temperature damaged the leaf oxygen evolving complex (OEC) and PSⅡ reaction centers.Conclusion Appropriate fertilizer dosage can improve photosynthetic capacity and alleviate the stress come from high temperature and strong light on walnut, while excessive fertilizer dosage is unfavorable to raise photosynthetic rate of walnut. It is suggested that the fertilization dosage should be kept at 612.8 g urea, 187.5 g P2O5, and 230.77g K2O per plant in the condition similar to the experiment site.