Abstract: According to Mohr-Coulomb failure criterion, the axial stress at failure in rock-soil triaxial tests is in linear relationship with the lateral stress. In this article, a group of formulae to calculate the cohesive strength c and the friction angle φ from rock-soil triaxial tests is obtained by solving the stress circle's envelope. The formulae have clear physical meanings and convenient applications. In addition, c and φ values can be respectively calculated by using such formulae so that another way to calculate shear strength parameters is proposed. According to the analysis, the cohesive strength c is proportional to σ_c (the axial stress at failure when lateral stress is zero), and decreases when K (the slope of the axial stress at failure versus lateral stress) increases; the friction angle φ increases with K. Using the formulae, rock-soil triaxial test data, which are representative in rock-soil engineering domain, are listed in this article.