Single cylinder axial confined piston hydraulic motor is a new type of dual power supply, through directly connecting plunger with piston pin and preserving traditional engine crank connecting rod mechanism, it can output hydraulic energy and rotating mechanical energy at the same time, and on the machine-liquid energy conversion, the power transfer chain is shortened, the energy loss is reduced.But single cylinder engine works unstabitily. It is easy to cause large flow pulsation of output high-pressure oil. The mechanical-hydraulic simulation model of single cylinder axial confined piston hydraulic engine was built by AMESim simulation software, and the plunger motion characteristics, pump cavity flow characteristics and output hydraulic oil pulsation characteristics in the mechanical-hydraulic power transfer chain were studied. The simulation results show that the plunger movement and pump cavity flow characteristics meet the design requirements of hydraulic engine and the flow pulsation of output hydraulic oil can be improved greatly through the reasonable selection of accumulator.