Abstract: To investigate the varation laws and characteristics of shear strength properties of red clay under different constraints, this paper takes red clay as the research object and conducts direct shear test and unconsolidated undrained（UU） triaxial shear test on remolded red clay specimens with different compaction degrees (K=80%, 90%, 95%) and water contents (w=15%, 20%, 30%). The results show that: under one-dimensional constraint, only horizontal shear failure occurs, with decreasing of water content and increasing compaction degree and confining pressure, the failure mode of red clay specimens under three-dimensional constraint changes from oblique shear failure to bulge failure or a combination of the two types of failures. The stress-strain curve of the specimen is “hump-type”, with significant stress softening characteristics. Under one-dimensional or three-dimensional constraints, the cohesion c index of the specimen is more significantly affected by compaction degree and water content than the internal friction angle φ, which is a strength index of the specimens.