The design of the good-performance materials for toxic formaldehyde (CH₂O)-gas-detection is critical for environmental preservation and human health. In this work, density functional theory (DFT) calculations were employed to investigate the adsorption behavior and electronic properties of CH₂O on transition metal (TM)-doped phthalocyanine monolayers. Our results prove that PdPc and RuPc monolayers are thermodynamically stable. Analysis of the adsorption energy showed that the CH₂O gas molecule was chemisorbed on the RuPc monolayer, while it was physisorbed on the PdPc nanosheet. The microcosmic interaction mechanism within the gas-adsorbent system was revealed by analyzing the density of states, the charge-density difference, the electron-density distribution, and the Hirshfeld charge transfer. Additionally, the RuPc monolayer was highly sensitive to CH₂O due to the obvious changes in electrical conductivity, and the recovery time of CH₂O molecule was predicted to be 2427 s at room temperature. Therefore, the RuPc monolayer can be regarded as a promising gas-sensing material for CH₂O detection.