The invention discloses a method and device for controlling a hydraulic system, and belongs to the technical field of hydraulic control. The hydraulic system comprises a diesel engine, a hydraulic pump, a forward direction hydraulic proportional electromagnetic valve, a reverse direction hydraulic proportional electromagnetic valve and an executing mechanism. The control method includes the steps that the actual rotation speed of the diesel engine and the moving direction of the executing mechanism are determined, and the moving direction of the executing mechanism comprises the forward direction and reverse direction; the linear relation of an opening control instruction of the forward direction hydraulic proportional electromagnetic valve and the actual rotation speed of the diesel engine and the linear relation of an opening control instruction of the reverse direction hydraulic proportional electromagnetic valve and the actual rotation speed of the diesel engine are obtained; and when the moving direction of the executing mechanism is the forward (or reverse) direction, the opening control instruction of the forward (or reverse) direction hydraulic proportional electromagnetic valve can be determined based on the actual rotation speed of the diesel engine and the linear relation of the opening control instruction of the forward (or reverse) direction hydraulic proportional electromagnetic valve and the actual rotation speed of the diesel engine. By the adoption of the method and device, the moving speed of the executing mechanism can be kept stable.