As a typical structure type of breakwaters,caisson breakwaters used to be constructed in the water area with good geologic conditions.With the extending of coastal structures to the offshore,deep-water and soft soil area,harsh ocean environment and soft geologic conditions put forward stronger requirements on the design theory and method of breakwater.Combining with the practical application,we firstly use the code method to compute the breakwater’s stability and get the most dangerous working condition;and then study the instability mechanism and failure modes by the finite element method under the conditions with and without shoulder stones.The results show that with the increase of wave force,when the cut-through plastic zone of soft clay foundation is formed,the caisson breakwater without shoulder stones slides first;the caisson breakwater with shoulder stone topples first,whose instability occurs in the form of rotation around a point below the bottom surface and on the right of the caisson axis,being different from the point of caisson hind toe in the code.