Nuclear energy is one of the most efficient, clean and stable energy sources. The technology of combined heat and power generation, which is widely used in the conventional thermal power plants all over the world, is also applicable to the nuclear power plants to make full use of the nuclear energy. But at present, the mode of combined heat and power generation is rarely used in the nuclear power plants in China, concerning about the nuclear safety problem. This study aims at the optimum use of nuclear energy to raise the potential margin of nuclear power plants, under the premise of ensuring nuclear safety, and analyzes the influence of the combined heat and power generation (such as the external supply of steam generated by nuclear power plants) of a pressurized water reactor on nuclear safety, including the influence on the normal operation, design basis conditions, design extension conditions and transient analysis. It shows that combined heat and power generation of nuclear power plants has no negative impact on nuclear safety. In the meanwhile, the mode of combined heat and power generation can improve the economy of nuclear power plants and has positive significance for the efficient use of nuclear energy.