Abstract: The open loop gain and some parameters are different on the two moving directions of symmetric valve controlling asymmetric cylinder system. This difference leads to dynamic performance asymmetry on the two moving directions. The natural nonlinear and asymmetric system is difficult to control. The last three joints of PБ-211 robot are controlled by symmetric valve controlling asymmetric cylinder system. To make the robot moving smoothly and improve the control precision, the control system must have proper compensation. After analyzing the asymmetric cylinder and taking the difference of protrusion and retraction into account, a new control algorithm, model reference self-tuning gain adaptive algorithm, was put forward. The new algorithm includes adaptive single neuron PID, model reference adaptive algorithm and self-tuning output gain. Simulations and experiments illustrate that the new control algorithm can compensate the asymmetric cylinder system, which is better than the command PID algorithm.