The invention provides a downhole while-drilling gas-logging testing device comprising a hydraulic module, a gas detecting module, a control module and a mud impulse generator. Through moving of a piston in the hydraulic module, annulus drilling fluid flows into the hydraulic module, the gas detecting module measures the content of methane gas in the annulus drilling fluid, and the concentration of the methane gas in a stratum is determined according to the measurement value; gas concentration data are transmitted to the control module, the control module processes the data transmitted by thegas detecting module and transmits the data needing to be uploaded to a ground system to the mud impulse generator, and the mud impulse generator converts data signals needing to be uploaded to the ground system into mud impulse signals and transmits the mud impulse signals to the ground system; and the ground system interprets the mud impulse signals and then obtains the uploaded data. Accordingto the downhole while-drilling gas-logging testing device, downhole gas parameter detection can be achieved in real time and online, and the accuracy of a tested result is improved.