The power balance (PWB) method is a fast method based on statistical electromagnetics for solving electromagnetic coupling problems in electrically large-sized cavity structures. Based on the PWB method, an electromagnetic coupling solver is developed to solve the electromagnetic coupling level of electrically large-sized cavity structure with different cavity shapes, different aperture shapes and different source types, etc. The validity and efficiency of the solver is validated by comparing its output with the results in the published literature and those of experiments.