Graphene quantum dots (GQDs) have both the properties of graphene and semiconductor quantum dots, and exhibit stronger quantum confinement effect and boundary effect than graphene. In addition, the band gap of GQDs will transform to non-zero from 0 eV of graphene by surface functionalization, which can be dispersed in common solvents and compounded with solid materials. In this work, the SnO2 nanosheets were prepared by hydrothermal method. As the sensitizer, nitrogen-doped graphene quantum dots (N-GQDs) were prepared and composited with SnO2 nanosheets. Sensing performance of pristine SnO2 and N-GQDs/SnO2 were investigated with HCHO as the target gas. The response (Ra/Rg) of 0.1％N-GQDs/SnO2 was 256 for 100 ppm HCHO at 60 ℃, which was about 2.2 times higher than pristine SnO2 nanosheet. In addition, the material also had excellent selectivity and low operation temperature. The high sensitivity of N-GQDs/SnO2 was attributed to the increase of active sites on materials surface and the electrical regulation of N-GQDs. This research is helpful to develop new HCHO gas sensor and expand the application field of GQDs.