CO2 and CO, the by-products of fossil fuels; one of them is a major cause of global warming and the other endangers the nervous and cardiovascular systems of humans. Therefore, real-time monitoring towards those harmful gases is of practical significance. Nano-structured materials have attracted the attention of scholars for their enormous potential for harmful gas detection. In this work, the adsorption and sensing behavior of C3B and Al-doped C3B monolayers for these two typical hazardous gases were investigated theoretically. The most stable doping model was obtained, and the adsorption process for CO and CO2 was simulated based on this model. The adsorption system shows that the gas molecules are all deformed and that the charge transfer and adsorption energy are significantly increased. Moreover, the adsorption mechanism was investigated by analyzing the electronic behavior of the adsorbent, and the physical adsorption between the hazardous gas and the adsorbent was more favorable for desorption. The good adsorption performance and sensing mechanism suggest that the CO/CO2 sensor prepared using Al-C3B has great potential for application. Our work may provide some guidance for the application of toxic gas monitoring and adsorption.