The pullulation ratio of the soybean,the content of the isoflavone for the cotyledon and laminae,and the mean height of the soybean seedling affected by a frequency doubled Nd:YAG 532 nm laser for different power density and time are presented in this paper. The mechanism is discussed from the activity of a phenylalanine ammonia-lyase (PAL) as an initial enzyme for synthesizing the isoflavone and the efficiency of the photosynthesis. The results show that the activity of the PAL and the products of the photosynthesis such as the soluble sugar,the sucrose,and the amylum are all increased after the soybean seed being irradiated by laser,in which the best result is occurred at 15 mW/mm2 of laser for 5 mins. At the same time,the efficiency of the photosynthesis of the soybean seedling is increased,which promotes the enormous accumulation of the isoflavone.