Abstract:A nonlinear model of pneumatic vibration isolation system was established. Based on this, simulation analyses were done using matlab/simulink software and the response curves were got under simple harmonic excitation. The results show that the system has strong nonlinear characters. Simple harmonic excitation can induce subharmonic/multiple frequency and offset response at the zero frequency. When frequency near to the linearized natural frequency the subharmonic and multiple frequency become stronger. Guided by the simulation results, a possible numerical solution was put forward. This solution which based on harmonic balance method got the numerical solution by using iteration method. The validity of this method is verified through comparisons withsimulation results, which show good agreements with the numerical solution.