Photoinhibition on soybean is studied, and the result shows that the photosynthetic rate of mesophyll cells is inhibited as the light intensity increased and the treatment time prolonged. The photoinhibition extent is different as the growth conditions different, it is easier to occur under conditions of low light intensity than that of high light intensity. The damage of photoinhibition is located mainly in PSⅡ. When the photosynthetic rate of leaves is photoinhibited, the quantum yield decreased. Phohoinhibition of photosynthesis is a reversible process. When it returns to low light intensity, however, the relative photosynthetic rate can not recover completely.