Abstract: To the nuclear primary pump of PWR and its two different primary piping cold leg diameter configuration schemes, a three-dimensional model is established by combining the primary pump and the primary pipe, and the hexahedron structured mesh is used to perform the model partition and calculate the unsteady flow characteristics of the whole flow region. The unsteady pressure fluctuation characteristics in the pump and the pipe under different cold leg configuration schemes of the primary pipe are obtained. The results show that the primary pump efficiency decreases with the increasing of the cold section diameter, but the hydraulic loss decreases with the increasing of the cold section diameter, and the efficiency of the whole system increases by 1.3%; the pressure fluctuation amplitude of transition section can be significantly reduced with the increasing of the cold section diameter; the difference of pressure fluctuation amplitude between the inlet position of guide vane and the pressure chamber in the two schemes of cold section diameter is small, and the amplitude of the pressure fluctuation in the cold section is also small, and all of them show the characteristics of no cycle and no rule; the configuration of large diameter cold section will slightly reduce the amplitude of the axial force fluctuation.