We took soybeans as experimental substance to study how the 632.8nm He-Ne laser affected the isoflavone content of cotyledon, laminae and stem. Results: the activity of PAL, the content of dissociated Phe and isoflavone are different with ck when embryo of soybeans were irradiated 1-7 min by laser with power density of 3.6 mW·mm
. The increase of dissociated Phe and the activity of PAL laid foundation for the great accumulation of isoflavone, and the optimal effect was observed at 5 min. The mechanism that laser pretreatment of soybean seeds could increase the isoflavone content of the seedlings was discussed in such both aspects as the activity of a phenylalanine ammonia-lyase (PAL) as an initial enzyme and the content of Phe.