Abstract: Based on the wave equation in unsaturated soil,the dynamic response of a pile under transient torsional loading was theoretically investigated.And the corresponding solutions of the displacement and velocity response function at the head of the pile were derived in frequency domain.Then,a semi-analytical solution for the velocity response of the pile subjected to a semi-sine wave exciting force was derived by using invert Fourier transform.The numerical results were obtained to analyze the influences of some major parameters on the response.It was shown that the present regressive solution was consistent with the solution of torsional vibration in saturated soil layer.