Lead (Pb2+) is a well-known highly toxic element. The mechanisms of the Pb2+ toxicity are not well understood for nitrogen metabolism of higher plants. In this paper, we studied the effects of various concentrations of PbCl2 on the nitrogen metabolism of growing spinach. The experimental results showed that Pb2+ treatments significantly decreased the nitrate nitrogen (NO3− − N) absorption and inhibited the activities of nitrate reductase, glutamate dehydrogenase, glutamine synthase, and glutamic–pyruvic transaminase of spinach, and inhibited the synthesis of organic nitrogen compounds such as protein and chlorophyll. However, Pb2+ treatments increased the accumulation of ammonium nitrogen (NH4+ − N) in spinach cell. It implied that Pb2+ could inhibit inorganic nitrogen to be translated into organic nitrogen in spinach, thus led to the reduction in spinach growth.