The authors intend to develop a navigation-warning system for small ship operators in severe sea. In this system, utilizing the wave information from the general marine radar and motion simulation method, ship operator can take the useful information about encountering waves and large ship motion in near future. To accomplish this system, we need to complete the two elements. One is to improve the wave radar, about this we already realize the practical system1), another is to develop the motion simulation method in time domain that can get the results in short time adequately for real time simulation. In this paper, we propose a practical simulation method that can calculate the large motion introducing the conventional knowledge based on the linear theory. Additionally some experimental results are shown for validation of numerical method. In conclusion, we present some merits and problems of this method and our future work to improve it.