As the dramatic advancement in array signal processing and the computer techniques, the phase array radar becomes more and more popular. However, the electromagnetic environment is more and more complicated with different kinds of interference. The ability to suppress the interference becomes the most important consideration for the phase array radar. In this paper, the mismatch optimal detector method is proposed to suppress the interference at the subarray level. In addition, the float round-off error analysis is given. The simulation results show that the proposed method has good performance in interference suppression.