Median control input, dead volume and leakage flow rate were identified for obtaining precise model parameters of pneumatic servo system. According to the physical phenomenon that the stable pressure values of port A and port B under the same control voltage would be different, the median control input of proportional directional valve could be obtained through minimizing the offset value of two stable pressure ratio curves. According to the physical phenomenon that the pressure responses of port A would be different but the stable pressure values of port A are identical when the pneumatic cylinder is fixed at two ends, the dead volume of rodless pneumatic cylinder could be obtained through equal leakage mass flow rate when equalizing pressure. In order to describe the steep mass flow rate just before the valve port was open and eliminate the noncontinuity between the flow rate equations of leakage and valve orifice, the leakage flow rate was transformed into the leakage throttling area that was fitted by a rational expression. The experimental results indicated that the model uncertainties are reduced, the control accuracy of the adaptive robust controller is improved, and the control input is very smooth through the identification of characteristic parameters and structure parameters.