Based on Maxwell's curl equations, the electric component and the magnetic component in magnetoelectric metamaterial plate are equivalent to the surface electric current and the surface magnetic current respectively. By calculating the total electric field and the total magnetic field in a magnetoelectric metamaterial plate generated by these periodic surface electric currents and magnetic currents, we obtain two equations about the surface current density and the surface magnetic current density, and thus deduce the analytical formulas for relationship between the refractive index of periodic magnetoelectric metamaterial and permeability of the magnetic component, and that between permittivity of the electric component and spatial dispersion. Unlike traditional index formula, the analytical formulas fully consider the spatial dispersion and the interaction of the electrical component and the magnetic component. The theoretical curves for refractive index are found to be in good agreement with the retrieval curves from simulation data, which shows that the analytical formulas for the refractive index can correctly describe the negative refraction characteristics of the magnetoelectric metamaterials. Our work will provide important theoretical reference for researchers to analyze interaction between electromagnetic components and to design magnetoelectric metamaterials with negative refractive indexes which meet certain requirements.