Magnetic-catalyzed SnO2 with Fe3O4 of CMOS MEMS gas sensor is proposed and it's based on the magnetic-catalytic sensing mechanism to increase sensitivity. Beyond the conventional power dissipation of heating to maintain a certain working temperature, a new approach for gas sensor with magnetic-catalytic mechanism works at the ambient temperature without the consideration of active heating. The design and fabrication is realized by the standard 0.35μm CMOS process to fabricate a gas sensor with mesh stacked electrodes. For the preparation of magnetic sensing material, a prepared solution of sol-gel SnO2 is mixed at SnO2:Fe 3O4 = 3:1, which was deposited onto mesh stacked electrodes. When the CO gas sensor is introduced, the sample is tested and verified inside a CO gas chamber with a magnetic field generator of solenoid coil. We also build a magnetic-catalytic gas reaction behavior description based on Gibbs free energy and the Eyring equation. A careful investigation of measurement results, at horizontal magnetic field, the sensitivity of proposed CO gas sensor reaches 1.73%/ppm under the 12 Gauss which shows widely applicable for an ultra-low power chemical microsensor with high sensitivity.