Abstract: The precision of degree measure is the key quota for monopulse secondary suneillancerada(MSSR), off boresight angle(OBA) indicates the relation between the sum-difference ration(SDR) of antenna and the departure for target with the point degree, so OBA is the key point for degree measure of target accurately. For the situation that original OBA radar data table could not be obtained directly and the antenna parameters could not be measured, degree measure of MSSR based on OBA adaptive build method is proposed in this paper, the basic principles and the implementation steps in the project is described in detail. The result that secondary radar actually works in the field shows that, the OBA table generated by the adaptive error statistics provides a more accurate correction data to extract the target orientation, and greatly improve the measurement accuracy of target position.