The tidal inlet in mud coast is of a special type. which was not studied in detail before. The paper studies the development of tidal inlet in mud coast, with the special reference to coast of Yellow River Delta. The studies show that the tidal inlet of this type has evolved from abandoned river mouth. The outer coast of the tidal inlet is shell sand bar created in higher sea level period and is reformed by current hydrodynamical processes. The global rise of mean sea level and the earth subsidence of the delta are important factors leading to the creation of tidal basin of the tidal inlet. The abandoned river channel changes into the main tidal creek in the basin. The scour against the creek system enlarges the drainage areas because the adjacent coast retreats at a greater rate after a river mouth is abandoned. Once the height of the shell sand bar is lower than that of mean sea level, tidal inlets of this type will start to decline. The present Disokou and Shenxiangou outlets are at a formation stage of the tidal inlet. Dakouhe and Taoerhe oulets are at a mature stage. The old Caofeidan oulet is declining.The paper points out another type of the declined tidal inlet in mud coast, which is because of depositon of tidal basin. Tlhe old Qikou outlet belongs to this type. The tidal inlet around nowdays Yellow River Dalta is compared with that of the abandoned Yellow River Dalta in North Jiangsu Province in this paper . It shows that along northern Jiangsu Coast, there doesn’t exist the welldeveloped tidal inlet because of human activity and the lower sea level.