In order to study the application of fractional calculus in the control of semi active air suspensions, a 4-DOF nonlinear dynamic model of semi active air suspension system was built. A modified-Oustaloup filter algorithm was adopted to simulate fractional calculus. The simulation model of semi active suspension with fractional-order sky-hook damping was developed. And its simulation results were compared with those of passive suspension and semi-active suspension with integral-order sky-hook damping. The comparison results indicate that in contrast to passive suspension, when the car is running at the speed of 20m/s on grade B road, the integral-order and fractional-order sky-hook damping control strategies reduce weighted RMS (root-mean-square) values of vehicle body vertical acceleration by 31.9% and 43.9%, and decrease RMS values of pitching angular acceleration by 23.1% and 30.7%. The results further show that the semi-active control based on fractional calculus could suppress vehicle body resonance more effectively.