Abstract: There are two methods to decide the interfacial shear strength of pile-soil in ocean engineering design nowadays. One is taking the advices of engineering experience and codes, the other is calculation using the strength parameters of soils derived from geological exploration. As for the pile foundation in ocean, it is very important to reasonably estimate the interfacial shear strength of soil-structure, which will make a great difference to the pile bearing capacity. Both overestimation and underestimation of the pile bearing capacity may lead to severe engineering accidents. To study the characteristics of interface between piles and soils during pile driving, a large-scale direct shear apparatus is developed to perform the direct shear tests on soil samples with varied particle diameters. The shear-resistant coarseness between the soils and the surface of structures is introduced, and the relationships among internal friction angle, relative density and shear-resistant coarseness of sand samples are studied as well as the relationships between the shear-resistant coarseness and the undrained shear strength of cohesive soil samples. According to the results of the tests, the shear-resistant coarseness is variable, and the shear-resistant coarseness of sand changes more regularly while that of soft clay changes more greatly owing to the influences of consolidation, disturbance and strength of soils. Moreover, some advices are offered to the application of shear-resistant coarseness in practical engineering.