In order to solve fault in the comprehensive performance hydraulic test rig under the load simulation condition,the output characteristic curves under different rotating speed were analyzed from the low speed stability of hydraulic power system,the occurrence mechanism of the fault was expounded through the establishment of mechanical characteristics of variablefrequency motor model.Combined with the characteristics of the dynamic system of the hydraulic test rig,the corresponding processing method was proposed,and its validation was carried out.The results show that the method can effectively solve the fault of the comprehensive performance hydraulic test rig.It has great guidance and reference significance to protect the safe and stable operation of hydraulic test rig.