The accurate computation of rock joint strength is important in practical engineering. To improve the accuracy of determining joint strength in Discontinuous deformation analysis (DDA), an improved joint strength model is proposed. The limitation of Mohr-Coulomb criterion with constant friction coefficient and the inconsistency of the definition of contact joint parameters in DDA are analyzed. In view of the existing problems, the improved joint strength model considers the different characteristics of two structure planes of a joint and friction angle degradation with the relative displacement between contact interface. And the specific implementation flowchart in DDA program is provided. The proposed model is examined by a case of block sliding under dynamic load. The results show that the improved model has the ability to determine joint strength consisting of two different structure planes and reproduce joint strength degradation.