Magnetorheological finishing is an ultra-precision machining process with stable removal efficiency and no subsurface damage. However， the water loss of magnetorheological fluid （MR fluid） in the polishing process will change the properties of polishing tools， thus affecting the stability of removal function. The existing water control strategy is affected by the large time delay and time-varying disturbance of MR Fluid circulation system， which leads to the periodic fluctuation of water content and the periodic time-varying removal function， thus affecting the machining quality and accuracy. In this study， the transfer function model of MR fluid circulation system was established， and the characteristics of the system were analyzed. based on this， a Model-free adaptive control based on full form dynamic linearization （FFDL-MFAC） algorithm is designed. The algorithm can realize the parameter adaptive control of nonlinear system， effectively suppress the water fluctuation caused by time-varying disturbance and delay， and provide a simple， effective and applicable control strategy for the stable control of water content in the polishing process. The experimental results show that when FFDL-MFAC control algorithm is adopted， the Peak-valley value （PV） of the water fluctuation of MR Fluid is only 0.06%， which is reduced by 40% compared with PID. The Integral value of absolute error （IAE） was reduced by 58.1%. The stability of water content of MR fluid in polishing process is effectively improved.