Current treatment for inhalational halogens poisoning involves providing supportive care, which includes administering humidified oxygen and managing the airway. Since toxic effects of halogens cannot be reversed, sensors with high sensitivity and good reversibility for detecting the relatively lower concentration yet noxious halogens becomes particularly significant and enticing. Herein, the structural and optoelectronic properties of toxic F2 and Cl2 gas molecules adsorbed on highly stable Al2SSe monolayer have been systematically studied by means of first-principles calculations based on density functional theory (DFT). Favorable adsorption sites of said molecules on Al2SSe were carefully examined. The relatively high, negative adsorption energy for F2 and Cl2 indicate that the adsorption process is exothermic and the molecules could be stably adsorbed on Al2SSe monolayer. This characteristic, combined with the substantial charge transfer (0.15-0.55 |e|), drastic change in work function, complete reversibility due to recovery time in 10-1 s scale and distinct optical response, render Al2SSe monolayer a viable option for utilization as either surface work functions transistor or optical chemical resistor for detecting these gases. Pearson correlation coefficient (PCC) analysis of theoretical recovery time and response value indicates that band gap change and electron transfer are the primary influencing factors. Selectivity analysis reveals that common compound forms of halogens and atmospheric molecules such as HF, HCl, N2, O2, H2 and H2O are either physically adsorbed or inert with extremely low adsorption energies on Al2SSe, prompting high F2 and Cl2 selectivity. These outcomes acclaim the exciting prospects of developing Al2SSe monolayer for specific, occupational related ultrahigh-sensitivity F2 and Cl2 sensing nanodevices.