The temperature field distribution of germanium (Ge) core fiber in the process of CO₂ laser annealing with different powers and different scanning speeds was simulated by COMSOL Multiphysics. The suitable laser annealing condition was obtained by analyzing the distribution and change of the temperature field in the laser annealing process. Combined with the Raman spectroscopy measurement of the fiber after laser annealing, the laser power of 2.153 W and scanning speed of 6 mm/s were proper for the Ge core fiber annealing when the outer diameter of the fiber was 190 μm, the inner diameter of the fiber was 28 μm. The simulation of the paper provided a reference for experiments to optimize the properties of Ge core fibers.