We propose and demonstrate a compact surface plasmon resonance (SPR) sensor based on an inner wall waveguide fiber (IWWF) for magnetic field sensing. The diameter of the central hole in the IWWF is 65 μm, which is propitious for metal deposition and infiltration of magnetic fluid (MF). We deposit a silver film on the inner surface of the IWWF by employing the silver mirror reaction method, and an evanescent field near the inner wall will excite SPR. MF is filled into the central hole of the IWWF, and the transmission spectrum will change with different magnetic field intensities by utilizing the tunable refractive index (RI) of the MF. The proposed sensor achieves a maximum sensitivity of 19.3 nm/mT in the magnetic field intensity range of 0.5-2.3 mT. The temperature sensitivity is measured to be -0.26 nm/°C in the temperature range of 25-50℃. Compared with other optical fiber magnetic field sensors, the sensor proposed in this paper has the advantages of simple structure, miniaturization, and high sensitivity, exhibiting great application potential.