The industrial toxic gases are becoming alarming for living lives day by day. The potential of boron nitride nanosheet (BNI) as a gas sensor for sensing industrially manufactured noxious gases such as COCl₂, CH₃NH₂, and H₂S has been investigated in this study. To determine the fixability as a gas sensor, the absorption behavior, electronics, and optical properties are examined using Density functional theory (DFT) calculation. From this investigation, it has been found that COCl₂ gas molecule absorbed strongly on the surface of BNI nanosheet rather than two other gases. The absorption energies E_abs of CH₃NH₂, H₂S, COCl₂ gas molecules are found in order of -0.2613205611 eV, -0.3218322304 eV, -1.845546685 eV respectively. This indicates that CH₃NH₂, H₂S gas molecules are low absorbent. Besides the bandgap of BNI nanosheet at room temperature is 4.765eV. After sensing with gas molecules, the bandgap is decreased in order of 4.328 eV, 4.664 eV & 3.002 eV respectively. In the case of optical property study, the conductivity, refractive index, absorption coefficient supports that BNI nanosheet is the best candidate for sensing COCl₂