Abstract: A side-heating formaldehyde sensor (SnO₂/MgO) was fabricated by doping SnO₂ with MgO. The UV light excitation method allowed the sensor to work at room temperature. The phase composition, microstructure and specific surface area of SnO₂ and SnO₂/MgO materials were characterized with the X-ray diffractometer, thermal field emission scanning electron microscopy and specific surface and porosity analyzer, and the performance of the sensor made of SnO₂ and SnO₂/MgO was tested under different annealing temperatures and different UV light irradiation intensities. The results show that the doped material has larger specific surface area and stronger adsorption capacity, and the sensitivity of SnO₂/MgO sensor has the best sensitivity when the annealing temperature is 650 ℃ and the UV light irradiation intensity is 1.75 mW/cm². When O₂ and C₂H₆O of the same volume fraction are used as the interference gas, it is found that the SnO₂/MgO sensor has good selectivity for HCHO. The research results provide a reference for the exploration of a new highly sensitive formaldehyde detection technology.