Using the rigorous coupled wave theory, the electro-optical diffraction properties of periodically poled lithium niobate with different parameters were numerically studied. The analysis show that, the periodic domain structures of a periodically poled lithium niobate act as an index of refraction diffraction grating under the applied electric fields and its diffraction characters have relation to the geometry structures, the period, the applied electric-fields and the incident angles. The periodically poled lithium niobate produces Bragg diffraction under the Bragg condition. In this case, the 0 order of diffraction light is periodically converted to －1 order of diffraction light with the increase of applying electric field. The maximum conversion efficiency can reach to 100% and the conversion voltage value can be reduced effectively by lengthening crystal. If angle of incidence is the Bragg angle, and the propagation of light in periodically poled lithium niobate does not satisfy the conditions of Bragg diffraction, the power of 0 order diffraction light can not transfer to －1 order of diffraction light with 100%. Meanwhile, the periodic modulation are damaged. The 0 order light is converted to corresponding multiple orders of diffraction lights with high diffraction efficiency when the incident angles are multiple Bragg angles. The maximum conversion efficiency is also reach 100%. The analysis results are helpful for developing the integrated devices of periodically poled lithium niobate.