Abstract: In order to study the changing law 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 UU triaxial shear test on remodeled red clay specimens with different compaction degree (K=80%, 90%, 95%) and water content (w=15%, 20%, 30%), respectively. The results show that: under one-dimensional constraint, only horizontal shear damage occurs, with the decrease of water content, compaction, the increase of the surrounding pressure, the damage pattern of the red clay specimen under three-dimensional constraint is changed from oblique shear damage to bulge damage or a combination of the two types of damage; the stress-strain curve of the specimen is “hump-type”, the stress softening characteristics are significant. The stress-strain relationship curve of the specimen is “hump-shaped”, and the stress softening is obvious; under one-dimensional or three-dimensional constraints, the cohesion c index of the specimen is more obviously affected by the compaction degree and water content than the strength index of the specimen's internal friction angle φ.