The vibration system of a geophysical prospecting car has three main features: the starting frequency is low (from 1Hz); the load is large (a single spring bears more than 3667Kg of load); the vibration signal is mixed with stochastic components. In this paper, models of an improved passive vibration isolation system and a semi-active fuzzy vibration control system of a geophysical prospecting car frame are established and simulated respectively. In the semi-active fuzzy control system the stiffness of an air spring is adjusted, and at the same time the impact by damping changes on the system performance is also taken into account. In order to evaluate the effects, the displacement and acceleration of car frame and the dynamic load of air spring are taken to comparative analysis. The results indicate that the proposed semi-active scheme can effectively lower the damage from vibroseis to the body of the car.