A side-hole fiber surface plasmon resonance (SPR) sensor is proposed and numerically analyzed to solve the cross-sensitivity problem of temperature and magnetic field. In the side-hole fiber SPR sensor, the side-holes I and II are filled with magnetic fluid and a mixture of ethanol–glycerin, respectively, resulting in a loss spectrum with two channels. Each channel has different sensing characteristics. When the external magnetic field intensity increases, the magnetic field sensitivities of channels I and II are 1.098 and −0.018 nm / Oe, respectively. Moreover, with the increasing temperature, the temperature sensitivities of channels I and II are −5.909 and −4.211 nm / ° C, respectively. Therefore, the variations of magnetic field intensity and temperature can be simultaneously measured by detecting resonant wavelengths of channels I and II of side-hole fiber SPR effects, resulting in the influence of temperature in a side-hole fiber SPR magnetic field sensor being eliminated. The side-hole fiber SPR sensor has obvious advantages of compact structure and high sensitivity, and it also has solved the problem of liquid storage glassware to compact the structure of the fiber SPR sensor further.