Abstract: Axial thrust balancing device is an important component for high-pressure multistage pump stable operations. In order to satisfy special reliability of structure and operation for high-pressure pumps used in reverse osmosis seawater desalination systems, a new kind of axial thrust balancing device was designed for multistage centrifugal pumps. Balancing device’s inner flow field and its mechanism were researched based on computational fluid dynamics method. Different axial and radial clearances’ performances as well as different length-diameter ratios were analyzed. The results indicate that inner flow field of sliding bearing clearance and support mechanism of bearing change greatly as inlet and outlet pressure conditions change; there is nonlinear relationship between balance force and axial and radial clearances, while there is linear relationship for length-diameter ratios and negative linear relation for relative leakages. A new device which axial clearance is 0.2 mm, radial clearance is 0.3 mm and length-diameter ratio is 1 is designed by above analyses. This optimized device can well balance the axial force and improve the reliability of multistage high-pressure centrifugal pumps obviously.