Formaldehyde is known as the common interior air pollutants, which can cause great damages to the human respiratory system and neural system with a small amount. Thus, it is necessary to monitor formaldehyde accurately and in real time at near-room temperature. In this work, the porous CuO hierarchical microspheres were synthesized by microwave-assisted hydrothermal method with various reaction time. As the reaction time prolongs from 1 to 7 h, the initial self-assembled CuO microspheres changes to nanoplates-coated microspheres. For CuO-5, the response of reaches 32 towards 80 ppm formaldehyde at room temperature and it also exhibits good selectivity as the response values to other test gases are lower than 5 These performances prevail over those of the other CuO samples in this experiment. The reason can be concluded that the exposed crystal faces and the active sites are different with increasing hydrothermal reaction time, resulting in different gas sensing properties. This work investigate the dependence of morphology and exposed crystal faces on gas-sensing properties in detail.