The study of the valid simulation method of the echo signal of the sea surface for synthetic aperture radar is one important part of the radar simulation system. Using the characteristics of the high frequency electromagnetic scatter, one method building the radar echo signal of the sea surface in the time region by the mean of Graphic Electromagnetic Computation is established, including the construction of the sea model, the computation of the echo filed strength and the echo time signal. Based on the principle of the Stripmap SAR, the results prove this method to be valid.