In this paper, a nonlinear model of pneumatic vibration isolator was established, considering the volumetric compressibility of air. Based on this, simulation analyses were done using matlab/simulink software. The response to two different types of disturbances is studied: single-frequency excitation and double-frequency excitation. Single-frequency excitation can induce harmonic frequency response and offset response at the zero frequency. When the frequency is near to the the linearized natural frequency, the harmonic frequency response become stronger. Nonlinear characters have much to do with the drive amplitude. When the drive amplitude become large, the nonlinear characters are more obvious. Double-frequency excitation can induce combination resonance. When the two drive frequencys are close, combination resonance are more strong.