In this paper, we find the decreasing law of maximum eigenvalue of the principal sub-matrix for coupling matrix, propose a method of calculating quickly pinning nodes in complex networks, and reveal the relation between the pinning strategy and the number pinning nodes. Numerical simulations show the trends of evolution under the conditions of three pinning strategies in a scale-free network and a small world, and the effectiveness of the pinning synchronization by selecting pining nodes randomly in a scale-free network.