Abstract: Considering the determination of mode effective refractive index in the free propagation region of AWG, a new method using weighted equivalent mode refractive index as the mode effective refractive index in the free propagation region was proposed. The vector wave equation of magnetic field in the free propagation region of AWG was demonstrated using Galerkin finite element method with absorbing boundary condition under the slowly-vary-envelope approximation. On this basis, the mode of free propagation region was analyzed based on 3D full-vector beam propagation method, and the equation of weighted equivalent mode refractive index was deduced with guided-mode propagation analysis method. This method provides a useful theoretical tool for the determination of mode effective refractive index in the free propagation region of AWG.