Abstract: Adding a difference antenna beam that has nulls in both azimuth and elevation on the basis of a traditional monopulse system will provide a possible approach for nulling of mainbeam jammers. The working principle of using four-hannel monopulse to counter radar active decoy is introduced, and its feasibility is analyzed. Then a method that using the phase difference between two different channels to realize angle tracking of radar active decoy is presented. At last its tracking performance is analyzed. The preliminary result shows that it is not only feasible in both theory and engineering, but also effective in the condition that the radar decoy return is not negligible. So four-hannel monopulse is believed to be an effective approach for countering radar active decoy.