Abstract: A new true tri-axial apparatus has been developed by Shao Shengjun et al working in Xi’an University of Technology. In order to measure the mechanics property of unsaturated soils under the condition of three dimension principal stresses, the airproof framework of sample was improved by changing the shape of sample’s latex membrane and bottom and top plate of sample. Through adding the measuring instrument of porous water pressure and air pressure, the metric suction of unsaturated soils can be measured in the process of true tri-axial compression of unsaturated soils. The true tri-axial experiment of remold loess with water content 18%, dry density 1.35 g/cm³ was carried out under the condition of controlling the middle principal stress ratio b. The stress and strain curves of remold loess were obtained respectively under different confined pressure ( =50 kPa, 100 kPa, 150 kPa and 200 kPa) and middle principal stress ratio (b=0, 0.25, 0.5, 0.75 and 1.0). Based on the Bishop effective stress shear strength formula, the matric suction parameter of remold loess with water content 21% was determined by the direct shear tests. Appling the Bishop effective stress formula, the true tri-axial effective stress state at failure was described respectively on the and plane. The shear strength changing with uniform effective stress is linear approximately.