To predict the failure load of masonry wall panel, this paper extracts the strain energy density from the finite element analysis (FEA) of masonry wall panel under the evenly distributed unit load and develops a concept of generalized strain energy density (GSED). Then, by analyzing the GSED modes of both base and new panels, and the relationship between the failure loads of two panels is revealed. Hence, a GSED-based method is proposed. The case studies show that the GSED-based method is much more accurate than the yield line method. Comparing with the FEA method, the GSED-based method has nearly the same accuracy but a low sensitiveness to different material properties. Besides, The GSED-based method can directly apply the experimental data to predict the failure load of new panel, which provides a new way for predicting task.