Abstract:The variable displacement pump-fixed displacement motor closed volume speed-modulating circuit which has many advantages such as linear speed output,constant torque output,high efficiency and high integration etc,is widely used in practical production.There is often an abnormal phenomenon that the replenishing pressure is too high or too low.If it runs for a long time,it will reduce the production efficiency,even damage the key hydraulic component.Based on the actual operating conditions,according to the incidence of the fault,it was sorted and the mechanism was analyzed and the corresponding solutions were proposed.It has certain guiding significance for practical personnel.