In combination with Lake-water source heat pump project in Nanjing, three-dimensional modeling was established to simulate the thermal environmental impact of warm/cold water discharge when the system is operating, and predict the range of warm/cold water discharge diffusion and the extent of temperature rise/drop. The results show that: the lake-water temperature rise/drop caused by the project meets the requirement of environmental quality standards for surface water. Affected by water density, thermal diffusion of discharged warm water is different from cold water and the discharged warm water has a greater impact on the temperature of surface lake-water and water temperature layering appear to be horizontal near the outfall area, while the discharged cold water has a greater impact on bottom water and the water temperature layering appear to be vertical near the outfall area. With the same emission load and the same emissions way, cold water discharge has greater impact on the water temperature of the lake.