In this paper, the feasibility of C-doped MoSeTe and the adsorption properties of several common toxic gases (SO2, H2S, Cl-2) on Janus MoSeTe and both side of MoSeTe-C monolayers were systematically investigated based on first-principle theoretical calculations. The results show that the MoSeTe-C monolayers has only weak interaction with H2S, and the adsorption energies of SO2 and Cl-2 are reduced to -0.547, -0.427 and -0.470, -0.475 eV on the Se/Te side, respectively, compared with that of the Janus MoSeTe monolayers . In addition, due to the high sensitivity of MoSeTe-C monolayers to SO2 and Cl-2 adsorption, the bandgaps of the developed systems are reduced by 37.9%, 36.4% and 44.3%, 53.2% on both sides, respectively. The results are critical to manifest the favorable sensing potential of MoSeTe-C monolayer upon SO2 and Cl-2, shedding light on the further explorations on MoSeTe-based materials for gas adsorption, catalytic and sensing applications.