Based on the theories of risk assessment and fuzzy comprehensive assessment, a new model of quantitative assessment to compute the risk degree of navigating ship on heavy sea was put forward.Using expert questionnaire method, the fuzzy relation matrix of occurring accident in different levels for a certain Ro/Ro vessel navigating on heavy sea was obtained, the risk degrees of different routes were calculated.The results show that on heavy sea the vessel faces a bigger risk degree on normal route, it has least risk degree on customary go-north route, it has a relative bigger risk degree on the compromise route of the two routes.It can be seen that the model can describe the risk degree of ship navigating on heavy sea quantitatively and objectively, it is useful for ship risky decision-making.