A large size visual direct shear testing apparatus is developed to carry out direct shear tests of geosynthetics and soil under various conditions. The loading modes and reaction system of the apparatus are improved to explore the interface properties and loading transfer mechanism of reinforcement soil. The front visualization of the shear box and automated data acquisition system are achieved to measure the strain of geotextile material. Direct shear tests are conducted on geogrid and coarse-grained soil by using the newly developed apparatus. The results show that the interfacial cohesion of reinforced soil increases, but its internal friction angle decreases compared with plain soil. The geogrid strain increases with the increase of shear displacement. Moreover, the rapid increase of embedded resistance between the geogrid transverse rib and the soil is caused by the rapid increase of the geogrid strain. However, the maximum strain of the geogrid generated by direct shear is far less than the yield strain of the geogrid. The direct shear interface thickness of plain coarse soil is less than that of the reinforced soil. The movement of coarse grained soil particles is mainly translational on the reinforced soil around the direct shear interface, and meanwhile some small particles are in the horizontal rotational movement mode.