Abstract：The variation of the strength of unsaturated soil with water contents is studied；and the practical strength formula is developed. To study the relationship between the strength and the water content，it is necessary to keep the water content constant in the test. The pore air pressure dissipates quickly and the pore water pressure does not have such behavior. To study the problem，it is necessary to guarantee that the pore air pressure has dissipated completely in the test，and to adjust the general triaxial test apparatus to strength tests of unsaturated soil. The test procedure and control measures to realize the above two test aims are put forward. Secondly，based on the test results，the influences of water contents on strength are analyzed. It can be easily found that the cohesion and the angle of internal friction of unsaturated soil decrease linearly with the increase of water content. Then，the total stress strength formula of unsaturated soil is established. If the water content distribution of soil strata is known，the strength can be easily obtained by the presented formula，i.e. the difficulty of measuring and calculating suction pressure can be avoided. For the water content can be easily obtained，the presented formula is convenient and practical.
