Best matching temperature is obtained when the fundamental laser is of normal incidence in periodically poled crystal. The in-depth analysis of second harmonic nonlinear conversion efficiency uses periodically poled crystal MgO∶sPPLT as the research object, and combines the Sellmeier equation and the polar period with temperature thermal expansion relationship. When the fundamental laser tilt is incident, the equivalent polar period becomes larger, and the best matching temperature is reduced. Acceptable temperature and wavelength bandwidth can be obtained conveniently by using the normalized frequency doubling efficiency curves for the given length of frequency doubling crystal. The crystal length becomes longer, and the acceptable temperature and wavelength bandwidth become narrower. As long as the Sellmeier equation and polar period with temperature thermal expansion relationship are known, the method used in this study can be extended to other periodically poled crystals. These conclusions are useful for other periodically poled crystals and guiding continuous wave fiber laser external cavity frequency doubling.