A 3-D parallel mechanism was designed to solve the problem of 3-D vibration isolation of vehicle seat. The kinematic analysis of the platform indicated that there was big workspace in vertical direction. The Jacobian matrix was not relative to the vertical displacement, so the parallel mechanism met the need of 3-D vibration isolation of vehicle seat. The results of mode analysis showed that three natural frequency of the 3-D vibration isolation system could avoid the sensitive frequency of body, and the dynamic sensitivity analysis for the 3-D vibration isolation system discovered the influences of the design parameter to the natural frequency.