Direct bonded periodically poled MgO doped lithium niobate (PPMgLN) ridge waveguide is a new wavelength converter with high conversion efficiency. The optical field distribution of the ridge waveguide is simulated by employing finite-difference method (FDM), the mode overlap of propagated waves in the ridge waveguide is calculated and the relationship between the overlap coefficient and the waveguide structure sizes is also investigated. The overlap coefficient to difference frequency generation (DFG) process conversion efficiency calculation is firstly introduced.