In this paper, we investigate an accurate and efficient method for nonlinear Maxwell's equation. DG method and Crank-Nicolson scheme are employed for spatial and time discretization, respectively. A semi-explicit extrapolation technique is adopted for the linearization of the nonlinear term. Since the proposed scheme is semi-implicit, only a linear system needs to be solved at each time step. Optimal convergent order of O(τ 2 + h p+ 1 2 ) is proved under time step size condition τ ≤ h d/4 . Finally, 2D and 3D numerical examples are provided to validate the theoretical convergence rate.