Abstract: The true triaxial apparatus is the key equipment to study the variation laws of the deformation strength of soils under the action of three-dimensional principal stress. It is also a test instrument for theoretical study of geotechnical mechanics. On the basis of vertical rigid and horizontal orthogonal two-direction flexible triaxis loading equipment developed by Xi'an University of Technology, the self-developed small true triaxis instruments with sample sizes of 70 mm×70 mm×70 mm and 70 mm×70 mm×140 mm are further developed into a large one with 300 mm×300 mm×600 mm with both upper and lower servo-controlled synchronous loading. The true triaxis instrument includes the main frame of the pressure chamber, vertical main stress loading servo-control hydraulic oil source, main stress loading servo-control hydraulic variable controller in the horizontal plane, sample pore water pressure servo-control hydraulic controller, and the servo-control and signal acquisition and control system. The loading equipment of the pressure chamber has a baffle with horizontal rotation along the vertical axis, radial rolling and expansion movement under hydraulic driving, which effectively separates the adjacent flexible hydraulic bag. In the development of the true triaxis instrument, special structure latex film and sample mold, axial plate rolling structure and PID-closed control optimization algorithm are used to solve the problem of sample preparation and installation, test operation, boundary influence and so on. The problems of the interference of the orthogonal two-direction flexible loading cavity in horizontal plane as well as the stability of the dynamic control system are solved. Finally, the large-scale true triaxis apparatus is used to verify the tests on remolded loess under the same value of b. The stress-strain characteristics and shear strength of the remolded loess are consistent.