The influence of temperature on the magnetic field sensor based on the magneto-refractive effect of an Er-doped fiber and an optical-fiber Mach-Zehnder interferometer is studied in this paper. In theory, the microstructure model of the Er-doped fiber material is established based on DFT. The structural parameters and atomic magnetic moments of the model are calculated at different temperatures. The numerical results indicate that as the temperature increases, the Er—O bond length and O—Er—O bond angle change, and the atomic magnetic moment increases monotonously, enhancing the magneto-refractive effect of the fiber. In experiment, an Er-doped fiber is used as the sensing arm of an optical-fiber Mach-Zehnder interferometer to fabricate the magnetic field sensor. The experimental results indicate that the magnetic field sensitivity increases with the increase of temperature. The sensitivities of the sensor to magnetic field are 12.63 pm/mT at 16.7 ℃ and 25.53 pm/mT at 43.5 ℃, respectively. The study on temperature characteristics of the magnetic field sensor is helpful to improve the accuracy of the sensor, and is of great significance to its practical application.