To improve gas sensing performance of SnO2 sensor, a heterostructure constructed by SnO2 and SnSe2 is designed and synthesized via hydrothermal method and post thermal oxidation treatment. The obtained SnSe2/SnO2 composite nanoparticles demonstrate a special core–shell structure with SnO2 nanograins distributed in the shell and mixed SnSe2 and SnO2 nanograins in the core. Owning to the promoted charge transfer effect invited by SnSe2, the sensor based on SnSe2/SnO2 composite nanoparticles exhibit expressively enhanced acetone sensing performance compared to the pristine SnO2 sensor. At the working temperature of 300 °C, the SnSe2/SnO2 composite sensor with optimized composition exhibits superior sensing property towards acetone, including high response (10.77–100 ppm), low theoretical limit of detection (0.354 ppm), high selectivity and good reproducibility. Moreover, the sensor shows a satisfactory sensing performance in trace acetone gas detection under high humidity condition (relative humidity: 70–90%), making it a promising candidate to constructing exhaled breath sensors for acetone detection.