In order to research the diurnal variation of excoecaria cochinchinensis Lour，the main physiological and ecological index under photosynthesis for its mature leaves were measured with LI-6400 portable photosynthesis determination system in october. The result showed that the diurnal process of photosynthetic rate was an obvious double-peak curve, the peak values were at 8:00 am and 15:00 pm. The photosynthesis had a significant midday depression phenomenon, and the main reason for this depression was the restrictions on stomatals.