The relationship and the dynamic changes of contents of proteins in different organs and three key enzymes such as nitrate reductase, glutamine synthetase and endopeptidase of nitrogen metabolism were studied based on one-year-old poplar clone 797 (Populus deltoides×P. euramericana) seedlings in this paper. The results showed that the contents of proteins in bark, xylem and root decreased obviously during the course of sprouting. The vegetative storage proteins mainly stored in bark and xylem in winter, and the contents in roots were relatively low at that time. The activities of nitrate reductase in leaves and roots reached to maxim in early Sept. and the middle ten days of Oct. respectively, and declined during defoliation. The activities of glutamine synthetase in leaves had two-peak curves, one was in the middle ten days of May, another was in Oct., the change tendency was consistent with the contents of proteins; and it reached to maxim in roots during sprouting, contrary to the change tendency of proteins; and they also declined during defoliation in both places. The activities of endopeptidase in roots rised gradually with the degradation of vegetative storage proteins during sprouting, and also kept rising during defoliation. There was distinct negative linear relativity between the contents of total proteins and endopeptidase activities in roots.