The hydraulic servo system generally regards the oil pressure as a constant，but the large flow condition and the pipeline effect in the long pipe will cause the oil supply pressure to fluctuate． These supply pressure chan-ges the result in servo system dynamic characteristic variation and thus degrade the control performance． To solve this problem，the changing oil supply pressure is estimated as an unknown parameter of the servo system based on the BackStepping control method． When Lyapunov functional derivative containing the estimation term isused，the principle must be negative and estimation method of the oil supply pressure is thus obtained． The servo system can adapt to the uncertain changes in supply pressure and keep precise position tracking performance． Experimental results show that the proposed estimation controller has higher position tracking accuracy when compared with the controller with good knowledge of supply pressure value．