A photonic crystal fiber sensor based on surface plasmon resonance was proposed for real-time detection of weak magnetic field. The surface of the photonic crystal fiber was covered with silicon nitride and silver film to achieve surface plasmon resonance, and the magnetic fluid on the outside of the sensor was used to sense the magnetic field changes. For the detection of magnetic field intensity, the performance indexes of magnetic field and sensor were summarized based on the performance indexes of refractivity. The simulation results of finite element method show that the wavelength sensitivity of the sensor can reach 0.55 nm?Oe^?1 in the magnetic field range of 80 Oe~1 000 Oe. The amplitude sensitivity, resolution, limit of detection, quality factor and fitting degree reach ?0.107 Oe^?1, 1.8×10^?2 Oe, 3.3×10^?2 Oe²?nm^?1, 6.73×10^?2 Oe^?1 and 0.991 17, respectively. The sensor also has the characteristics of simple production process and easy to manufacture, which can foresee its broad prospect in magnetic field detection.