The dry state critical speed of the rotor of the high speed multistage centrifugal pump of 3×8 3/4－10stg HSB was first calculated by means of the method of finite element and ANSYS software. After that, the effects on the critical speed of the rotor impressed by the support stiffness, gyroscopic couple, fluid softening effect, and mouth ring support, were computed respectively. Finally, the effect of the axial stress on the critical speed of the rotor was computed. The results showed that critical speed of the immersion rotor still existed deviation when only considering the fluid-solid coupling effect, and that considering the effect of the axial stress on the rotor dynamic characteristics the first critical speed was improved by 21% based on the fluid-solid coupling, and the ones of other order were also improved. It illustrated that the axial stress had an important effect on the critical speed of the rotor system and could improve the rotor stability.