Abstract: Using CIRAS-2 portable photosynthesis system, we measured the photosynthetic parameters of Prunus sibirica L. under eight soil moisture conditions in loess hilly-gully region to investigate the impact of soil moisture on the photosynthetic diurnal changes. The results showed that: 1) The reasons for photosynthetic midday depression of Prunus sibirica L. changed with different soil moisture conditions. When relative soil moisture content (W_r ) was in the range of 56.1%-66.1%, the phenomenon of photosynthesis midday depression does not occur. But beyond this range, the phenomenon would occur in different soil moisture levels. When W_r was in 93.2%-77.0%, the main reason of photosynthesis midday depression was stomatal limitation. As W_r was in 49.3%-37.3%, the midday depression was changed from stomatal limitation turned to non-stomatal limitation, when W_r less than 37.3%, the main reason of photosynthesis midday depression was non-stomatal limitation, which means the photosynthetic capacity of mesophyll cells declined. 2）When W_r was in the range of 49.3%-77.0%, Prunus sibirica L. had a higher photosynthetic rate and water use efficiency.