Abstract: Abstract: In view of water-lubricated bearing-rotor system's safety, reliability and stability of the high-pressure multistage centrifugal pump in the process of start caused by the transient unsteady motion of journal, this paper explored the multistage centrifugal pump's start-up and transient axis trajectory through the theoretical analysis and experimental research. First of all, on the basis of the finite element model of the multistage centrifugal pump's water-lubricated bearing-rotor system, the differential equation of dynamic movement under the action of outer incentive was established; then we combine the previous dynamic equations of the high-pressure multistage centrifugal pump's water-lubricated bearing-rotor system with the small parameter algorithm of solving the water-lubricated bearing's dynamic characteristics, and establish the transient coupled dynamic model of high-pressure multistage centrifugal pump's water lubricated bearing-rotor system. Based on this model and Newmark β algorithm, we can solve iteratively and numerically to gain the transient axis trajectory of the water-lubricated bearing-rotor system in the start-up phrase through the MATLAB programming techniques. Finally, we use the designed and developed special water-lubricated bearing test rig of desalination high-pressure pump to research the start-up and transient axis trajectory of desalination high-pressure pump's water-lubricated bearing by experiments, and then compare the measurement results with theoretical calculation and analyze their similarities and differences. From the overall comparison results, in the beginning of start and in the unstable phrase of the liquid film formation, the journal presents upward movement and moves toward the direction of rotation movement under the action of transient liquid membrane force; in the later stages of the start-up phrase and as the rotational speed increasing, journal presents "whirling motion" under the small amplitude. In the process of the establishment of the theoretical model in this article, we assume that the mass eccentricity of discs such as impeller, balance disc and so on are the same, and also ignore the shaft's mass eccentricity and the vibration of the outside world (such as the vibration of the machine run time, fluid pulsation); but these simplified or ignored factors exist in the actual experiments, and we also should consider the installation precision of sensors. So, the former axis trajectory is more smooth and the latter's is more cluttered. In conclusion, the overall trend of the transient axial trajectory in the startup phrase by measurement and the theoretical calculation are basically identical. So, the calculation model has a certain theoretical and engineering application value proposed in this paper, and provides a certain research base to further improve the starting performance of the 10 000 ton desalination high-pressure pump system.