This paper presents a semianalytical approach for evaluating the collision risk between two moving surface ships. The concept of probability flow is introduced to develop an analytically sound problem formulation, which allows for an accurate estimation of collision probability considering time-varying ship trajectory uncertainties. For efficient computation, the flow of collision probability is separated into diffusion and drift components. These two probability components are combined to obtain the instantaneous collision probability, and this instantaneous probability is integrated in time to quantify the expected risk of collision. To demonstrate the feasibility of the proposed approach, traffic simulations are performed for several representative maritime traffic scenarios and the obtained simulation results are discussed.