Lens array is always used to improve the uniformity of far field spatial intensity distribution on a high power laser system. This article introduces the simulated annealing algorithm into an optimal design of lens array, which changes the size and lens attributes of array elements, and finally reduces the ununiformity of the focal speckle from 0.2268 which is measured with a classical design to 0.1202 under the condition that the energy efficiency is satisfied, this shows that the optimal design greatly improves the uniformity. The curves of power spectral density of two speckle are also contrasted to research the impact of different design structures on it.