Abstract: A method to calculate modal radiation reactance of a simply supported rectangular panel and the detailed procedures and formulae of calculation are described in this paper.The method expands an existed approach on a fast evaluating of modal radiation resistance.The proposed method significantly improves calculation speed by reducing one dimension of integral in comparison with the former traditional approach,where the modal radiation impedance is often expressed by a double integral.Furthermore,the current method differs from the former traditional one,since its calculation time doesn't increase with the growth of the modal number.This method can be widely applied to the coupling problems of sound and vibration between structures and heavy fluid.