The sum-difference four-channel mainlobe interference cancellation technique for monopulse radar at subarray level was studied. To this end, the signal model of four-channel interference cancellation at two-dimensional subarray level for monopulse radar was established, and a quantitative method using the signal-to-noise ratio loss of the useful signal as an indicator to characterize the angular resolution of mainlobe interference cancellation was proposed. Based on this, an analytical expression for the angular resolution of interference cancellation was further derived, and the theoretical boundary of the cancellation angle blind zone was clarified. The correctness of the analytical formula is verified by simulation and anechoic chamber experiment results. This research can provide a reference for the theoretical boundary analysis and engineering implementation of four-channel mainlobe interference cancellation technology.