The Guanzhong Plain of Shaanxi Province is one of the major regions of wheat production in China. Farmers believe that the more nitrogen is applied, the higher yield is obtained. In fact, the excessive application of nitrogen fertilizer does not increase the yield of wheat synchronously, but cause a series of environmental issues. A simple but effective technique for crop nitrogen estimation can lower the production cost and improve the ecological environment. The critical nitrogen concentration can be used to diagnose the crop nitrogen status. Previous researchers mainly developed the critical nitrogen dilution curve based on the aboveground dry matter, whilst the leaf dry matter was used to establish the curve in this study. The field experiments were conducted between 2013 and 2015 to construct and validate the critical nitrogen dilution curve based on leaf dry matter in the vegetative stage in winter wheat. In this study, we selected six winter wheat varieties and set four nitrogen levels N0（0）,N1（105kg/hm2）,N2（210kg/hm2） and N3（315kg/hm2）. Results showed that there were negative power relationships between the critical nitrogen concentration and the maximum leaf dry matter. The nitrogen nutrition index derived from the critical nitrogen dilution curve could be used to evaluate the nitrogen nutrition status of winter wheat. The nitrogen nutrition index increased with the nitrogen rate, with the values between 0.57 and 1.21. The relation between nitrogen nutrition index and relative yield indicate that 105~210kg/hm2 was the optimal nitrogen rate for winter wheat in the Guanzhong Plain. The critical nitrogen dilution curves based on leaf dry matter were successfully used to assess of the nitrogen nutrition status of winter, which can be used for guiding scientific application and diagnose of nitrogen in Guanzhong Plain.