Abstract: With the large-scale of horizontal vessels, the saddle stress can not be ignored, especially under extreme load, the risk of bolt breaking is greater. In order to prevent bolt breaking, this paper introduces a calculation method of saddle stress, focusing on the calculation method of polar moment of inertia of web and rib plate assembly, so as to maximize the bearing capacity and reduce unnecessary waste of resources, At the same time, the method of reasonably setting anchor bolts is also given to improve the bearing capacity of web, rib plate and bolt assembly.