Suboptimum performance may be obtained by processing at subarray level with a low cost in large array. Aiming at uniform linear array(ULA), a subarray level processing method is proposed. Considering the impact of subarray division and weight vector at subarray level on array performance, subarray partition and weight vector at subarray level are optimized by particle swarm optimization (PSO) at the same time. Simulation results show that co-optimization method can make full use of the array dividing degrees of freedom. When the array output performance is given, the proposed method can get the way to divide the array and subarray amplitude weights easily. Compared with the conventional method, co-optimization method can reduce the computation amount of array designing and the cycle for designing array. Moreover, pattern with greater pertinence can be achieved. The method also provides a theoretical basis of subarray partition for a large array.