Non-Equilibrium Green's function (NEGF) and density functional theory (DFT) method is employed in exploring the geometric, electronic and adsorption aspects of phosgene (COCl2) on phosphorene nanosheet based molecular device. Further, the attributes such as Bader charge transfer, energy band gap, and average energy gap variation assists in scrutinizing the surface assimilating properties of COCl2 gas on phosphorene nanosheet. Besides, the detection properties of COCl2 gas molecules using the phosphorene nanosheet device are evidenced by the transmission spectrum and V-I characteristics. Thus, the current work establishes the foundation for employing phosphorene nanosheet based molecular device in sensing COCl2 gas molecules.