The structural behavior of reinforced concrete frame infilled with a masonry wall is investigated by the method of discontinuous deformation analysis (DDA). An interface element is developed and it is incorporated into DDA to analyze the continuous and discontinuous behavior of the masonry structure. The numerical results are compared with previous research and possess satisfactory agreement. Then the structural behavior and stress distribution of a reinforced concrete frame infilled with a masonry wall subjected to a horizontal force are studied. In addition, the justification of equivalent strut is assessed by the distribution of principal stresses. The results show that the behavior of the masonry structure is highly influenced by the failure of mortar. On the basis of the distribution of principal stress of the masonry wall in the reinforced concrete frame, the equivalent strut can be approximately substituted for the masonry wall without separation and opening. However, the application of equivalent strut to the masonry wall with separation and opening needs further study.