Abstract: PIPESTRESS program is used to analyze and evaluate the nuclear safety class 1 and class 2 piping model with elbows, and the influence of two calculation methods of the separate calculation and the restart calculation is compared. The research shows that there is a deviation in the result of the separate calculation. In order to get the result that meets the standard requirements,the restart calculation should be adopted. This paper can provide a reference for the piping calculation of generation Ⅲ and Ⅲ nuclear power plants.