Abstract: It is necessary to take mutual coupling of arrays into account in the research of array pattern synthesis for Opportunistic Array Radar (OAR) with a large number of antenna elements randomly distributed in the space. Active element patterns of a large array are constructed from those of a small array. In this paper, based on the idea of the sub-array, the large array is divided into across some sub-arrays, which can include the same elements. Building the mutual impedance matrix?of?large arbitrary array with the mutual impedance matrix?of the sub-array and combining with adaptive algorithm to realize the beam pattern synthesis optimization for OAR. The method of moment is used to calculate the generalized mutual impedance matrix of sub-arrays in this method. Last, based on maxim output signal-to-noise rate criteria, adding imaginary jammers into angle domain to adaptively change weights of the array factors for synthesizing patterns. An uniform linear array and a plane array are synthesized with the method. Calculated results agree with that with FEKO Software Simulation.