In order to further increase the power density for axial-piston pump, the quantitative formulas were derived and the most important factors were studied. One factor was the amount of volumetric displacement of pump per revolution divided by the spatial volume of pump, and the analysis indicated that it was determined by swashplate angle and piston pitch-radius, and 60% increase of it could be obtained by using an inverted piston-slipper design. The other factor was the maximal speed which was determined by the self-priming ability (SPA) of pump. The analysis result showed that the SPA was affected by resistance caused by viscous resistance and forced vortex, and the ability dropped with the rotating speed increasing, which was the defect of current structure of cylinder block. In order to overcome the defect, a new structure of cylinder block with bidirectional inclined cylinder block hole was presented, and the results of simulation demonstrated that SPA of axial-piston pump was increased by 45.4%. Finally, to meet the requirement of large displacement pump, a symmetrical X shape axial-piston pump was proposed and its power density was verified theoretically. The analysis result showed that the power density was increased by 43% compared with a double-pump.