In order to improve summer trimming techniques of mulberry trees and minimize its influence on the growth of mulberry trees, the author made a study into the physiological function of remaining leaves of mulberry trees after summer trimming of Husang32, Xinzhiyita and Yunsang2. Results indicate that chlorophyll percentage and photosynthesis increased rapidly and reaches the highest value 4 or 5 days after trimming, and then decrease gradually.