Abstract: AP1000 is the advanced third-generation pressurized water reactor nuclear power plant (NPP). To ensure the safety of NPP under accident condition, it is necessary to study the characteristics of the main piping of the secondary loop when double-ended break condition occurred. The RELAP5/MOD3.4 code was used to carry out numerical simulation of the NPP secondary loop under double-ended break condition. The calculation results were obtained, which included the spray flow rate, pressure, void fraction, dynamic thrust force and other parameters of the piping breach, as well as the changing characteristics of those parameters. The calculation results of RELAP5/MOD3.4 were compared with the results calculated by ANSI 58.2 simplified algorithm. And it turns out that the dynamic thrust forces derived from the calculation results of RELAP5/MOD3.4 are smaller than those of ANSI 58.2 simplified algorithm. The analysis of this paper lays out the basic foundation for piping whip restraint design due to piping break for AP1000 NPP.