A special testing apparatus was designed for measuring and calculating fluid pressure in pump chamber and balance cavity, fluid pressure in front-back pump chambers was measured and analyzed by changing balance hole diameter and using a pressure sensor. It was found that liquid pressure in back cavity was generally bigger than that in front cavity under normal conditions of seal ring，and the curves of fluid pressure in front-back pump chamber above the diameter of seal ring were different. Based on systematical analysis of pressure curve of pump chamber with or without fluid leakage, the computational formula of fluid pressure in pump chamber with fluid leakage was presented by introducing pressure loss coefficient under the design condition. The fitting formulas of pressure loss coefficient were given by calculation of experimental data under the design condition. Moreover, the theoretical calculation results and the experiment results of fluid pressure in pump chamber with fluid leakage had also been contrasted and analyzed for two centrifugal pumps. The results showed that the computational formula of fluid pressure in pump chamber with fluid leakage had high credibility. Fluid pressure in balance cavity was calculated using mathematics model under the design condition. The experiment results proved that this mathematics model could accurately forecast fluid pressure in pump chamber. In order to control the fluid leakage and reduce the axial thrust, it was proposed that the balance holes diameter of 6~8mm was appropriate through analyzing the theoretical calculation results and experimental results.