A special testing apparatus was designed for measure and calculation of fluid leakage in flow passage of pump chamber. Gap-diameter ratio was changed by changing axial position of the impeller, i.e., changing the axial clearance of the pump chamber, when gap of wear-ring was 0.2mm and 0.3mm,respectively, and length of wearring was 15mm, the import and export liquid pressure and fluid leakage in flow passage of pump chamber (gap-diameter ratio was 0.127, 0.101, 0.076, 0.051, 0.025 and 0.006, respectively) were tested and analyzed. The curves of leakage coefficient and pressure coefficient were obtained under different gapdiameter ratios conditions. In addition, the curves of velocity coefficient and gapdiameter ratio with different pressure coefficients were provided by calculating the experimental data. Results showed that under different gap-diameter ratios, fluid leakage coefficient and pressure coefficient in flow passage of pump chamber varied regularly, their relation curves were almost oblique lines, but gapdiameter ratio and gap of wearring can significantly affect curves. When the structure of pump was constant, decrease of axial clearance of pump chamber can effectively reduce fluid leakage, thus the volume efficiency of pump can be increased, and the optimum value range of axial clearance of pump chamber was 1~5mm. Computational formula of fluid leakage in flow passage of pump chamber and determination method of velocity coefficient were presented. This study is of great significance to the volume efficiency of pump, wheel resistance loss, liquid pressure distribution in pump chamber and the calculation of axial thrust.