Abstract: In order to explore the photosynthesis response mechanism of crabapple seedling to high temperature, the growth and photosynthesis of three crabapple varieties during the growing season and photosynthetic diurnal variation in high temperature period were examined. The study provided the theoretical basis for promotion and cultivation of crabapple varieties. The results were as follows: high temperature stress caused net photosynthetic rate and the growth rate of three crabapple varieties decline, there was significant difference among them. The heat-tolerant variety remained faster growth rate and the decline of its net photosynthetic rate was lighter than weak heat-tolerant variety, and occured in the late of high temperature phase, while weak heat-tolerant variety occured in the early of high temperature phase. Their diurnal variation curve of net photosynthetic rate was a typical double peak type, and are obvious “midday depression of photosynthesis” phenomenon, but the main reason of this phenomenon was not caused by stomatal closure.