A method for controlling a constant-pressure fluid enables an inverter with a build-in controller to control rotation speed of a pump to achieve constant-pressure control thereof. In the control method, a pressure feedback value of output flow of the pump is measured and is compared with a pressure reference value to produce an error pressure value. Afterward, an operation condition of the pump, such as an abnormal handling condition, a deceleration standby handling condition, or an acceleration condition, is determined according to the pressure error value to shut down or re-start up the pump after a delay time. Hence, the control method can avoid frequently starting up or shutting down the inverter. Therefore, the power consumption and the operation cost are reduced. The use life of the pump prolongs and constant-pressure control for the pump can be implemented.