The sensing measurement of magnetic field has an important application in related fields. Using the magneto-optical effect of magnetic fluid, a magnetic field sensing structure based on optical Tamm state is proposed. The structure is composed of a one-dimensional magnetic fluid photonic crystal loaded with a metal layer and a dielectric layer. The influence of the structure parameters on the sensing performance is studied numerically. The results show that the thicker is the magnetic fluid layer, the higher is the detection sensitivity. The thickness of the metal layer and the dielectric layer both have an optimal value, so that the sensor has higher detection accuracy. The results also show that the detection sensitivity of the proposed sensing structure is better than that of the reported magnetic field sensor realized by photonic crystal defect structure. The research results provide a reference for the design and fabrication of magnetic fluid magnetic field sensors based on optical Tamm state.