Transition-metal nitrides (TMNs) have attracted increasing attention due to their outstanding hydrogen evolution reaction (HER) performance. However, the effect of nitrogen content on HER performance has been rarely studied. Herein, the multi-composition CoMoN is synthesized using conventional hydrothermal and nitridation treatments. The HER performance of CoMoN improves initially with increasing urea content and then basically remains unchanged. When the urea content is 2.4 g, CoMoN only needs the overpotentials of 37, 164, and 578 mV to achieve current densities of 10, 100, and 500 mA cm, respectively, with a long-term stability of 110 h at high current densities. Ex situ results analysis reveals surface reconstruction during HER, leading to the formation of Co(OH) and CoMo, with nitrogen promoting α-Co(OH) formation. In situ Raman spectroscopy indicates that the metals play a primary role in HER. This study provides valuable insights into optimizing nitrogen content for HER, which may have significant implications for industrial applications.