The growth of soybean leaf can be divided into three stages: rolled leaf, unrolled leaf, shaped leaf. The growth rate of rolled leaf was speeded up with the increase of air temperature, but it was no longer increased when air temperature was over 28℃. The date at which the amplitude of risen air temperature＞the growth speed of rolled leaf, showed that high temperature promoted growth and branches occured. It was about 13-14 days from the unfolding of trifoliates to the time when it falls into a shaped pattern. This suggested a duration of the responsibility to photoperiod of soybean. The responsibility to photoperiod of unrolled leaf promoted the occurance of primary branches, and that of a pattern-shaped leaf promoted the occurance of secondary branches.