This paper deals with the reasonable (protective) application of nitrogen fertilizer for soybean.It was by means of the ‘nitrogen-factor effect' of wheatstraw with a high ratio of C/N to regulate the immobilization mineralization process of soil or fertilizer nitrogen.The biological effect of protectively applied nitrogen on modulation N-fixing activity,nitrogen forms in Xyloid liquid,and amount of assimilated nitrogen in soybean were studied.Results indicated that protectively applied nitrogen could decrease even eliminate the inhibiting effect of nitrogen fertilizer on nodulation and N-fixiing activity of soybean,condinate the supply of nitrogen from nodule and soil and increase the relative content of ureide-N and the content of organic nitrogen in its xylon.The normal development and function of symbiotic nitrogen fixation system would be protected.It is beneficial for the synthesis of soybean protein. Therefore,as 20kg/hm² and 40kg/hm² of nitrogen fertilizer were applied,N in the straw of soybean increased by 5.1% and 23.0% and N in the seed increased by 23.0% and 32.6% respectively with protectively application of nitrogen.