Since the successful synthesis of graphene, two-dimensional materials, including hexagonal boron nitride and transition mental dichalcogenides, have attracted wide attention due to their extraordinary properties and extensive applications. Recent researches have revealed that the sensing system based on graphene or MoS₂ can efficiently sense various gas molecules. However, the utility of these materials is limited by their inherent weakness, i.e. the zero bandgap in graphene and the relatively low mobility in MoS₂, which impede their applications in electronic devices. This further stimulates the motivation of researchers to find more novel 2D materials. Black arsenic phosphide (AsP) monolayer, a novel two-dimensional nanomaterial with the characteristics of model direct bandgap and superhigh carrier mobility, is an ideal material for gas sensor. Here in this work, we investigate the electronic and magnetic properties of monolayer AsP absorbed with small gas molecules by using first-principle calculations based on density functional theory. Four initial absorption sites are selected to explore the optimal absorption positions of CO, CO₂, NH₃, SO₂, NO and NO₂ absorbed on the monolayer AsP. The purpose is to calculate the optimal absorption configurations, the absorption energy, absorption distance, and charge transfer, thereby investigating the absorption types. The results revel that the monolayer AsP is sensitive to NO₂ gas and SO₂ gas via strong physical absorption, and NO gas by chemical absorption, forming a new bond between N atom and O atom. The CO, CO₂ and NH₃ gas are absorbed on AsP monolayer with weak van Waals force. From the point of view of charge transfer, the CO, CO₂, and NH₃ molecules are one order of magnitude smaller than SO₂, NO and NO₂, approximately 0.03e and the charge transfer of NO gas is 0.21e, highest in all gases. Besides, the effects of absorption on the electrons of AsP are investigated. The results show that the absorption of CO, CO₂ and NH₃ molecules have little effect on band structure, and that the absorption of SO₂ molecule increases the bandgap. The absorption of magnetic gas NO and NO₂ reduce the bandgap by introducing impurity level near Fermi level, giving rise to their magnetic moments of 0.83μ_B and 0.78μ_B and making the whole system magnetic. Theoretical research shows that monolayer AsP is sensitive to NO, NO₂ and SO₂ gas molecules, which provides theoretical guidance for the experimental preparation of gas sensors band on black arsenic phosphorus.