This paper proposes an optimized simulated annealing (SA) algorithm for thinning and weighting large planar arrays in 3D underwater sonar imaging systems. The optimized algorithm has been developed for use in designing a 2D planar array (a rectangular grid with a circular boundary) with a fixed side-lobe peak and a fixed current taper ratio under a narrow-band excitation. Four extensions of the SA algorithm and the procedure for the optimized SA algorithm are described. Two examples of planar arrays are used to assess the efficiency of the optimized method. The proposed method achieves a similar beam pattern performance with fewer active transducers and faster convergence ability than previous SA algorithms.