Based on the impact of nonlinear factors on the actual mechanical properties of inerter, a nonlinear mechanical model with friction and elastic effect of screw was built. The mechanical property testing of inerter was carried out and the parameters of inerter nonlinear mechanical model were identified through the test data. Then, the mathematical model of two-stage series-connected ISD suspension with five degrees of freedom and nonlinear inerter was built. The optimized variables were determined through the sensitivity analysis of ISD suspension parameters. According to the performance evaluation index of ride comfort, the parameter optimization mathematic model of ISD suspension was established and the parameters were optimized by particle swarm optimization (PSO) algorithm. The optimization results indicated that under the premise the optimization met the requirements of driving safety and suspension working space, and the root mean square value of weighted seat vertical acceleration went down by 24.12%. It was concluded that the method proposed could effectively improve the vehicle ride comfort. 