The test by changing different diameters of balanced hole in the 3BA—6 single grade single suction pump was made. The pressure of outlet of impeller rear sealing and the pressure of inlet in pump were measured. The test curves of the liquid pressure in balance cavum and the pump head, and the pressure of import of the pump and the pump head were both obtained. Thus the fluid leakage characteristic curves of balance cavum were obtained. The theoretical curves and the test curves of fluid leakage in balance cavum at the different diameters were compared and analyzed in design condition. After measured the axial force curves of pump, the result showed that axial force curves which obtained by calculating based on the fluid leakage characteristic curves of balance cavum could meet the engineering calculation need.