In order to achieve a higher volumetric efficiency of an internal gear pump at higher pressures,a new isolation device with a compensating function in the diameter direction was developed.The device used a technical method of expanding high pressure chamber inside the pump.In order to obtain a better balance for the isolation device,according to the pressure distribution law of the transition zone,the calculation formulas of hydraulic pressure,the compensation force and the supporting force of the crescent block in the transition zone of the crescent block were derived,and then the equilibrium constraints of the device were established.The transition zone was enlarged until only two gear teeth.As an example,the fmincon tool was used to obtain the optimum center angle of the compensation cavity,and the force changes curve was obtained by MATLAB simulation.When an internal gear pump with a theoretical displacement of 50.8 ml/r was carried out under the condition of input pressure of 0.3 MPa,output pressure of 21.8 MPa,at 50℃ ±4℃,and the results show that the volumetric efficiency can reachs 94.3%.No abnormal wear was observed.