Abstract: The ground surface soils are almost unsaturated, especially in arid and semi-arid areas, and the engineering problems of unsaturated soils cannot be properly explained and dealt with by the saturated soil mechanics and the relevant experimental means. First the unsaturated degrees of soils are described by low and high suctions, respectively. By reviewing the theory of saturated soil mechanics, the current commonly accepted theories of unsaturated soil mechanics are introduced. Then, the testing methods for measuring the water retention and mechanical behaviors of unsaturated soils in the low suction range are presented. The focus is on the precautions and countermeasures required by the use of ceramic plates and the volume measurement method for unsaturated soils by using the triaxial instruments. The results of the suction-controlled compression, triaxial shear and true triaxial tests are used to illustrate the typical mechanical behaviors of unsaturated soils. After analyzing the coupling characteristics of hydraulic and mechanical behaviors and the factors directly influencing the water-retention curves based on the test results, a coupling model for describing the hydraulic and mechanical behaviors of unsaturated soils is established by the using the elastic-plastic method, and is verified by suction-controlled or measured isotropic compression and triaxial test results. Subsequently, the stress-strain relationships of four typical soils over a wide suction range are given by means of the suction-controlled triaxial test method developed by the author's group in the high suction range. Finally, the measured strength data of two soils over a wide suction range are used to analyze the applicability of various equations for calculating the strength of unsaturated soils.