Abstract: Nonlinear waveguide is an important photon quantum device. Using Marcatili approximation, mode field distribution characteristics of waveguide can be approximately described. For periodically poled LiNbO3 waveguide with ee-e quasi-phase matching, fundamental mode field distribution of fundamental wave with a wavelength of 1 064 nm and second harmonic of 532 nm is analyzed. Results show that if waveguide is an embedded square waveguide, fundamental and second harmonic are basically circular, and range of fundamental wave field is larger. When length of square side of section is 5 μm and core area is slightly 0.02 higher than cladding refractive index, waveguide can basically confine two kinds of light field in core region, but constraint ability becomes very poor when waveguide length is 0.5 μm. If difference between core region and cladding layer is large, waveguide with 0.5 μm side length can get two kinds of field distribution constrained in core area basically. Results are of significance to analysis of modes of other types of nonlinear waveguides.