With the unceasing development of the traffic tunnels in subway and highway engineering, sometimes the tunnels pass through rivers or lakes, so slurry shield method with large diameter is often used in the underwater tunnel construction. The underwater tunnel with large diameter has the characteristics of high risk, difficult to control, more uncertain risk factors, which will cause greater risk in construction process. Risk assessment is more and more applied in underwater tunnel construction safe management. The characteristics of using slurry shield with large diameter in underwater tunnel were introduced in this paper, and the mechanism that how the characteristics arouse the increase of risk was analyzed. According to the disadvantages of previous risk assessment methods, an improved index method was put forward, and the assessment process and main calculate model of this method were given. This method has been used in an underwater tunnel in Nanjing subway engineering, the application results show that this method can adapt the dynamic change of risk factors and it can be better applied in underwater tunnel construction safety risk assessment.