Abstract: A photonic crystal fiber sensor based on surface plasmon resonance is proposed for real-time detection of weak magnetic field. The surface of the photonic crystal fiber is covered with silicon nitride and silver film to achieve surface plasmon resonance. Magnetic fluid is used to sense the magnetic field change. Based on the index of refractive index, the performance indexes of magnetic field and sensor are summarized. The finite element method results 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 figure of merit reached −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 manufacturing process which can predict its broad prospect in magnetic field detection.