Abstract: According to Drucker-Prager criterion, considering the influence of the intermediate principal stress on the width of the plastic zone in the coal wall, the ranges of the plastic zone and the force bearing status are studied, the mathematical formula of the plastic zone width and stress expression which can reflect the stress distribution in the plastic zone are deduced, and also the calculated results are compared with the results deduced from the classic Mohr-Coulomb criterion derivation formula. At the same time, the major factors for the width of the plastic zone are discussed in detail. The results prove that the intermediate principal stress has significant effects on the width of the plastic zone in the coal wall and shows the interval of effect. The calculated results indicate that the Drucker-Prager criterion derivation formula is safer than the Mohr-Coulomb criterion derivation formula when used in engineering practice.