Abstract: Hoek-Brown criterion has been widely applied in rock engineering. However, there still exists a defect in the elastoplastic rock constitutive model proposed on the basis of this criterion. A plastic flow rule based on a piecewise implicitly modified plastic potential is proposed here when the feature of the plastic deformation development of rock under different confining pressure is considered. This method can solve the plastic flow discontinuity problem in other constitutive model using generalized H-B criterion and avoid the additional input parameters as well. Based on the modified Euler mid-point type constitutive integration method, the numerical implementation of this model has been conducted, with the result verification on single integration point level. This model has also been attached to the numerical platform of GeoFBA3D, and further results verification in a classical elastoplastic mechanics problem as well as a typical application example in tunnel engineering are also developed.