Echo removal is an important way to improve the spectral resolution of terahertz time-domain spectroscopy. In order to study the echo removal method for the cases of nonlinear absorbent samples, the absorption and group delay effects were considered based on de-convolution algorithm. Two of the four parameters needed in conventional algorithms were abandoned and the remaining two are not related to the exact material and thickness of the sample, which improves the accuracy of terahertz spectrum recovery. Results show that the presented algorithm can eliminate echoes generated by GaAs antenna and ZnTe detection crystal effectively. The width of the time window is doubled so that the spectral resolution is improved up to 20 GHz.