In order to avoid throttling loss and recover differential pressure energy, braking energy and gravitational potential energy without changing the original hydraulic system, the fourport hydraulic transformer with inlet and outlet equal flow(FHT), which can be directly connected to the original hydraulic system, is proposed. First, the basic structure and principle of the FHT are explained, the main characteristic of FHT is that there are four waist shape slots of the same size and shape in the valve plate, and the pressure ratio can be changed by adjusting the control angle of the valve plate, and the flow rate of port A is the same as port B, the combination of port A and B is equivalent to a hydraulic motor, the flow rate of port O is the same as port T, and the combination of port O and T is equivalent to a hydraulic pump. Then, the mathematical models of displacement, angular torque and pressure transformation ratio about the fourport hydraulic transformer are built. Moreover, the relationship between pressure transformation ratio and flow of the lowpressure port, recycling pressure difference and oil viscosity are obtained by using the method of partial derivation, based on this, the influence characteristics of the three factors on pressure transformation ratio are got by MATLAB simulation. Finally, the result of theoretical analysis and simulation shows the increase in flow rate of load circuit results in decreasing the pressure transformation ratio, while the decrease of oil viscosity and the increment of recycling pressure difference result in increasing pressure transformation ratio.