The sidelob e performance of synthetic impulse and aperture radar(SIAR) is discussed, and particularity of such system is pointed out. The analysis of the conventional process shows that the direction of arrival(DOA) of muti interference is a difficult p roblem for SIAR system . Therefore,a novel method combining focused beam forming and eigenvalue deco mpositio n is presented，and the DOA performance is researched based o n the experimental data. Result shows that the direction of the interference can be measured accurately by using this method. The work is important to desig n and application of the SIAR system.