In the future ultra-span and ultra-capacity optical telecommunication systems, high speed electro-optic polymer modulator is one of the key components, which is an all-fiber component with low insertion loss and high integration. The modulation efficiency can be improved because this kind of modulator can greatly suppress the mode mismatching and operate at much lower modulation voltage. An electro-optic polymer modulator based on the long period grating inscribed in a silica-based optical fiber is modeled as a five-layer uniform circular waveguide, and simulated by using the compact supercell method with the consideration of the complex refractive index of the electrodes. The mode index and mode pattern can be obtained, and can be used to explain some differences of the waveguide properties or the modulator characteristics between with and without the electrodes.