Abstract: In order to study the rotor dynamic characteristics of the double-blade submersible sewage pump, the ANSYS-CFX and ANSYS-Workbench are used to conduct numerical analysis of the natural frequency, mode shape and critical speed of the rotor system of a certain type of submersible sewage pump in dry and wet mode based on fluid structure coupling. The results show that most of the rotor vibration modes are swing and torsional oscillation; the first two orders are symmetric left and right oscillation of the impeller, and the maximum displacement occurrs in the impeller rim area and blade outlet area. The natural frequency of submersible sewage pump rotor in wet mode is lower than that in dry mode, with a minimum drop rate of 2.8%, which appears in the second order; the maximum drop rate is 21.3%, which occurrs in the sixth order. The critical speed of rotor in wet mode is slightly lower than that in dry mode. The working speed of the rotor is much less than the first four orders critical speeds of the rotor. It indicates that the rotor system would not resonate when operating at working speed.