A quantitative understanding of the pattern of coastal change is an essential prerequistite for the reasonable design of coastal engineering projects. Such an understanding is of course based on the correct recognition of physical background of the coast. Mud beach has its unique morphological feature and changing pattern, which differs from sandy coast. This paper, taking the mud beach as a studying object and putting emphasis on the dynamical background and changing pattern of the present coastal process, establishes a development model of coast erosion under natural conditions and engineering conditions and shows the theoretical maximum erosion profile. This model is proved by practical erosion profile in the Northern Jiangsu coast. The achievement of this study provides a scientific basis for the engineering protection of erosive mud beach and gives an example for establishing the theory of equilibrium profile in mud beach.