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In order to reveal the efficiency characteristics of axial piston pump and improve the accuracy of efficiency characteristics mechanism model under the whole working conditions, a semiempirical parametric modeling method based on mechanical forward and backward inference for piston pump efficiency characteristics was proposed. Considering the volume loss caused by oil compressibility and the piston pair coulombs friction formed by overturning moment, the flow and torque loss mechanism models were established. Then the nonlinear changing regularity of system parameters contained in the composite parameters of mechanism models was analyzed and the composite parameters empirical formulas were fitted based on measurement data. The piston pump efficiency characteristics semiempirical parametric model was obtained. The results show that average flow and torque loss estimation error are respectively within 3% and 1% over wide operating ranges of speed 500~3000r/min and pressure 2~24MPa. Composite parameters and corresponding loss energy proportion reflect the efficiency characteristics of piston pump directly. The rapid changes of system parameters, such as oil effective volume elastic modulus and coulombs friction coefficient, are the fundamental reasons of volumetric efficiency and mechanical efficiency unable to upgrade again and the piston pump efficiency decreased under extreme conditions. The models established in this paper can be used to investigate energysaving optimization design, performance evaluation, performance degradation and failure mechanism analysis of piston pump deeply and realistically.