The approximately analytical solution for a semiactive suspension system under relative control with timedelay is investigated in this paper. At first, the quartercar model is established and dimensionless analysis is done. Then a system of four nonlinear algebra equations for the approximate solution is obtained by the averaging method, which could be easily treated by numerical methods for algebra equations. At last, the numerical solution of the semiactive relativecontrol suspension system is also obtained through the simulation in MATLAB software. The comparison of the two methods shows the approximately analytical solution is correct and has satisfactory precision.