A gas detecting device includes a casing, at least one gas transporting actuator, at least one valve and at least one external sensor. The easing has an airflow chamber, an inlet, a branch channel and a connection channel. The airflow chamber communicates with an environment outside the casing through the inlet, and the branch channel communicates with the airflow chamber and the connection channel. The gas transporting actuator is disposed within the branch channel for transporting air into the airflow chamber and the branch channel from the inlet, and has a nozzle plate, a chamber frame, an actuator, an insulation frame and a conducting frame. The valve is disposed between the connection channel and the branch channel for controlling the air to flow into the connection channel. The external sensor is detachably disposed within the connection channel and has a sensor for measuring the air in the connection channel.