A vehicle gas-detecting device includes a control unit, a gas-detecting unit, a warning unit, and a power-connecting unit. The gas-detecting unit is electrically connected to the control unit. The gas-detecting unit detects the concentration of a gas in a vehicle. The warning unit is electrically connected to the control unit. The power-connecting unit is electrically connected to the control unit and a power source of the vehicle. Thereby, when the concentration of the gas within the vehicle is greater than a predetermined threshold, the control unit activates the warning unit to give a warning to draw users' attention.