Abstract: Shepard's and Folk et al.'s classifications are commonly used in the study of detrital sediments. Although both of them are based on the sediment grain size, the conception and classification standards of classification are very different. We have used two classifications to study the surface sediments in Tianjin Sea. The results show that, comparing to the Shepard classification, Folk et al.'s diagram is more simple and practical. The sediment type distribution and hydrodynamic pattern, given by the Folk et al.'s in the working area, are consistent with the local dynamic conditions, and show a clear sedimento-dynamic significance. To a certain extent, therefore, the Folk et al.'s classification reflects the changes in sedimentary environments, provenance and transport directions. This successful case study, illustrating the rationality of Folk et al.'s classification, enables us to recommend it in the marine sedimentology in order to obtain better presentation for sedimentary characteristics and distribution patterns.