Sapindus mukorossi Gaertn. (Indian soapberry) is an important tree species with many economic and ecological applications. In recent years it has been widely planted in poor mountainous areas of southwest China in association with forest restoration programs. At the same time, purple soil acidification due to excess use of fertilizers has increased in the southwest hilly areas and this has impacted the growth of S. mukorossi. Photosynthesis leads to creation of plant organic matter which tends to accumulate, but when photosynthetic capacity declines there are progressive impacts on soil quality. Studying the responses of photosynthesis and growth of S. mukorossi to acid soil stress will provide a theoretical basis for effective planting. We conducted soil acid stress experiments using randomized block experimental design in a purple soil acidic region of Chongqing. Three-year-old saplings were examined growing in soils having different degrees of acidity (pH 4.4, 5.6 and 7.6). The effects of soil acidity on growth, leaf traits and photosynthesis were analyzed. The results showed that the contents of soil available nitrogen (AN) and available phosphorus (AP) in the neutral zone were significantly lower than those in the acidic zone, and the contents of total nitrogen (TN) and total potassium (TK) in the leaves of the neutral zone were significantly higher than those in the acidic zone (P<0.01). Compared with neutral soils, in acidic regions the height of saplings, net photosynthetic rate of leaves (P_n), total chlorophyll content (Chls), chlorophyll a (Chl a), carotenoid (Cars), and ratio of chlorophyll a/chlorophyll b (Chl a/b) were significantly lower (P<0.01). The maximum photochemical efficiency of PSⅡ(F_v/F_m) and the PSⅡnon-photochemical fluorescence quenching coefficient (qN) were also lower (P<0.05). In addition, the content of chlorophyll b (Chl b) and the ratio of chlorophyll/carotenoid (Chls/Cars) were significantly higher (P<0.01). Crown width, PSⅡelectron transfer rate(ETR), PSⅡ photochemical fluorescence quenching coefficient(qP) and actual primary light capture efficiency (Y(II)) also increased (P<0.05). In strongly acidic soil, the light saturation point (LSP) of leaves was significantly lower (P<0.01), while the apparent quantum yield(AQY), light compensation point (LCP) and dark respiration(R_d) were the highest. AQY, LCP and R_d were lowest in the weakly acidic soil. Ground diameter, specific leaf area (SLA), leaf area (LA), stomatal conductance(G_s)and transpiration rate(T_r)of the experimental saplings in the weakly acidic soil were the highest among the three experimental soils, and significantly higher than in the strongly acidic soil (P<0.05). The acidity of purple soil significantly reduced the photosynthetic capacity of S. mukorossi. in acidic region by affecting soil nutrient transformation and root absorption capacity. The saplings in weak acidic region showed strong ecological adaptability owing to increasing light energy capture area, increasing photosynthetic electron transfer rate and light energy capture efficiency of PS II, and reducing organic matter consumption. We conclude that the suitable range of pH values for planting Sapindus mukorossi in purple soil regions of Chongqing was 5.5-7.5, while growth was poor when planted in purple soil with pH less than 5.5.