To understand the effects of different nitrogen fertilizer on the composting spent mushroom substrate of Pleurotus eryngii, we added Potassium nitrate, ammonium sulfate or urea into the compost separately and then measured their carbon–nitrogen ratio (C/N). The results showed that compost with addition of urea or ammonium sulfate got higher temperature, lower pH, higher oxidation–reduction potential (ORP) value, and higher maturity than that of compost with potassium nitrate. In the process, the content of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), ammonium nitrogen (NH4+–N), nitrate nitrogen (NO3––N) and nitrite nitrogen (NO2––N) in the compost could change, the content of DOC and DON in the compost with potassium nitrate were higher than those of the compost added ammonium sulfate or urea after composting. This comprehensive analysis showed that addition of different nitrogen fertilizer could affect the temperature, pH, ORP, which together resulted into the different maturity of the compost.