In order to study the relationship between the instantaneous speed fluctuation of hydraulic motor and the hydraulic system efficiency under the condition of constant load and changing rotational speed, the experimental verification method was adopted. Through setting the magnetic powder brake load voltage and changing the motor speed with the LabVIEW software in variable speed hydraulic experiment, the condition of constant load and changing rotational speed was simulated. The change curves about the hydraulic motor rotating speed fluctuations, the efficiency and pressure of hydraulic system, were collected and analyzed under the condition of constant load and changing rotational, such as ramp, sine and step. The experimental results show that the hydraulic motor instantaneous speed fluctuation and hydraulic system efficiency is relevant; the higher the speed is, the smaller the speed fluctuations of the hydraulic motor is, the higher the efficiency of the hydraulic system is; on the other hand, the change is opposite. Because of the changed speed, the system pressure is changed with a small range. The tendency of the pressure is same to the rotational speed and the system efficiency, and is opposed to the speed fluctuation. It provides reference for choosing appropriate hydraulic motor speed range, decreasing speed fluctuation, and improving efficiency of hydraulic system under the condition of constant load and changing rotational speed.