This paper presents the optical limiting and its dependence of the geometrical arrangement in the focused optical system by employing the Gaussian decomposition method.The results show that optical limiting in near field arrangement is very different from that in far field arrangement,and the nonlinear materials with positive nonlinear coefficient are not suitable to be used as the optical limiting materials.Meanwhile,the analysis also shows that the sensitivity of Z scan technique with the focused optical system is higher than that of common Z scan technique.