To explore the feasibility of arsenene in detecting H₂S gas, we employ the density-functional theory to investigate the geometry, electronic structure and magnetic properties of defected and doped arsenene. Point defects do not appreciably improve the sensing performance of arsenene due to small adsorption energies and charge transfer. The doping of transition metals (Ti, V, Cr, Mn, Co and Ni) introduces magnetic moments and narrows the band gap of arsenene. Transition metal (TM) dopants can enhance the interaction between H₂S and a modified arsenene substrate. Adsorption energies and charge transfers increase significantly, and the adsorption converts to chemisorption. After adsorption, the Ti and Cr-doped system's band gap change is twice that of the pristine and defective arsenene. The adsorption of H₂S changes the system properties of two TM-doped arsenenes: Ti-doped arsenene transforms from semiconductor to half-metal, and Ni-doped arsenene transforms from half-metal to conductor. Electrical signals can be observed in this process to detect H₂S molecules. Our calculations show that doping improves the detecting performance of arsenene to H₂S molecules more efficiently than defects. Our results indicate that arsenene has a promising future in developing H₂S gas sensors.