Based on the frequency doubling mechanism of a continuous wave 1560 nm laser single-passing a PPLN crystal, the dependence of nonlinear conversion efficiency on temperature is investigated. The effects of thermal expansion of the crystal′s polarization period on the nonlinear conversion efficiency and matching temperature have been studied in detail. By using finite element method, the temperature fields in the whole crystal and on the output facet are numerically calculated, and the nonlinear conversion efficiency is extracted when the laser passes through the crystal. The dependences of the crystal size, input laser power, and the strategy of heat dissipation on the temperature field are also presented, respectively. It is demonstrated that the performance of the PPLN crystal on frequency doubling can be improved by a wider crystal and the materials with good heat conductivity surrounding the crystal.