Abstract: Aiming at the estimation problem of the Direction of Arrival (DOA) of weak targets in the presence of strong mainlobe interference for digital sub-array radar, this paper proposes a sum-difference beam angle measurement method. Firstly, a receiving model of the mainlobe interference and target echo under the multi-subarray sum-difference beam structure is established. Then, according to the uncorrelated characteristics of the target echo and the interference signal in the time domain, blind source separation is used for each sub-array to separates the target echo and interference signal, and monopulse angle measurement technology is used to achieve target/interference DOA estimation via combining the target echo/interference signal separated by each sub-array. Finally, the effectiveness and analysis performance of this method is verified by numerical simulation experiment.