Taking HP multistage pump as example, physical model and mathematical model of its bearing-rotor system were established though appropriate simplified process. Four stiffness coefficients and four damp coefficients which express its dynamic characteristic of the water membrane bearing were deduced and calculated. Rigid support Campbell drawing by multi-load modal analysis on ANSYS considering inertial effect was generated. The fore four natural frequency considering inertial effect and critical speed on the load of 3000r/min were also obtained. Unbalance response curves of the shafting on working speed and unworking speed were obtained by analysis. This water lubricated bearing-rotor system was certified to be a steady rigid system by transient state characteristic analysis which driving force was feed in a sudden and the rotational speed was considered. Though rather large vibration may appear in the beginning of this period, oscillating amplitude will be reduced gradually and return to stabilization soon.