C3N monolayer is reported with comparable and even more desirable sensing behavior upon small gas molecules in comparison to graphene. In this work, we proposed Pt-doped C3N monolayer as a desirable 2D sensing nanomaterials for NOx detection by DFT method. The Pt atom tends to be doped on the BC-C site of C3N surface, making little effect on the bandgap but changing the indirect semiconducting property on the other hand. Pt-C3N monolayer behaves more admirable performance upon NO2 adsorption than NO, but both systems are identified as chemisorption with Ead of -2.25 and -1.99 eV, and QT of -0.344 and -0.083 e, respectively. The metallic property is determined in both systems given the calculated zero bandgap, which will increase the electrical conductivity of Pt-C3N monolayer largely after adsorption of NOx gases. This is the basic sensing mechanism for Pt-C3N monolayer upon NOx detection. In the meanwhile, the desirable changes of WF and dielectric function in NOx systems verify the potential of Pt-C3N monolayer for NOx detection through field effect transit or optical devices. Our calculations could be meaningful to exploit novel 2D nanomaterial for sensing NOx in order to monitor toxic gases in our environment.