The optical magnetic field sensor has the characteristics of small size, light weight, good insulation characteristics and has a wide application prospect in the field of magnetic field detection. In the research, it is found that the stability of magneto-optical crystal in small magnetic field is poor, and there are obvious fluctuations, which seriously affects the magneto-optical sensing performance. In order to study the sensing characteristics of magneto-optical crystals in a small magnetic field, this paper establishes a model of crystal magnetization from the perspective of microscopic magnetic domains, and analyzes the magnetization process of the crystal. Finally, this paper proposes a method to improve the sensing performance of the crystal in a small magnetic field by adding a bias magnetic field.