Abstract: The reinforcement of soil can be regarded as an equivalent confining pressure. Through analyzing Mohr's circle, the increase in the minor principal stress is obtained. Assuming that the soil is in a limit state at failure, a limit equilibrium equation is developed to calculate the strength of the reinforced soil while the cohesion and the internal friction angle of the reinforced soil change simultaneously. The formula is discussed in detail, the major principal pressures of the six kinds of reinforced soils under triaxial compression loading are calculated, and it is found that the calculated major principal pressures are entirely consistent with those obtained by the tri-axial tests.