In this paper, we introduce a three-dimensional triaxial apparatus with rigid walls. Its pressure chamber comprises four sliding rigid plates, a rigid specimen cap, and a rigid bottom plate. It has a three-dimensional servo hydraulic load control system, an intelligent control and data storage system, and a water-air suction control system. Considering a cuboid soil specimen as a true triaxial shear layer and a vertical principal stress transfer layer, the vertical principal stress is transferred from the transfer layer to the shear layer, and the orthogonal horizontal principal stress is applied by the horizontal slip rigid plates. That solves the technical problem of mutual interference observed in conventional three-dimensional rigid plate loading. The L-shaped loading plate is improved, which reduces the deflection and friction between them. Linear guides ensure that the horizontal stress is applied synchronously and the specimen is always centered during a test. True triaxial testing of Xi'an loess is reported, and the results confirm the applicability of the apparatus in soil testing.