To investigate the calculation models for shear strength of expansive soil under conventional and low stress conditions, as well as the effects of moisture content and dry density. Laboratory direct shear tests were conducted using expansive soil samples from the Mengzi region, Yunnan Province. The results indicate that under conventional stress conditions, the stress-strain relationship of expansive soil exhibits strain-hardening behavior, while strain-softening characteristics were observed under low stress conditions. Through systematic analysis of experimental data, two distinct Mohr-Coulomb shear strength equations incorporating moisture content and dry density were established for conventional and low stress conditions, respectively, with validation results demonstrating high prediction accuracy. Significant differences in shear strength parameters were identified between low and conventional stress conditions. The analysis reveals that moisture content exerts a greater influence on cohesion than internal friction angle under both stress conditions. Furthermore, the reduction rate of shear strength parameters with increasing moisture content shows a positive correlation with dry density. These findings provide valuable references for engineering construction in expansive soil areas.