Tri-axial test is one of effective means to measure the strength of unsaturated soil. But because of complexity and exquisite requirement with and long duration of test, unsaturated tri-axial test wasn’t used extensively in civil engineering before. The tests of shear strength of unsaturated soil are performed with the improved tri-axial apparatus for unsaturated soil. According to test data, the relationship of strength between water content and matrix suction are studied. In addition, the new shear strength formula suitable for yellow river is derived. The formula is the theory basis for analysis of unsaturated slope stability.