Combining magnetic catalysis effect, surface plasmon resonance phenomena, and common-path heterodyne interferometry, a highly sensitive and highly stable carbon monoxide sensing technique was proposed in this paper. Magnetic catalysis material SnO2-Fe3O4 was prepared and was spin coated on the gold film of SPR prism. The SPR apparatus can provide a highly sensitive ability to carbon monoxide. Carbon monoxide gas at different concentrations was measured under different strengths of applied magnetic fields. The experimental results reveal that the system has a best resolution of 0.0507 ppm applied 24 Gauss magnetic field in normal direction. Due to the introduction of common-path heterodyne interferometer, the system is stable against environment vibrations.