A gas detecting device includes a main body, a gas sensing module, a particulate measuring module and a control module. A chamber is formed within the main body. The main body has a first inlet, a second inlet and an outlet in fluid communication with the chamber. The gas sensing module includes a compartment body, a carrying plate, a sensor and an actuator. The actuator introduces ambient gas into the gas sensing module through the first inlet, and the gas is measured by the sensor and discharged from the outlet of the compartment body. The particulate measuring module is disposed within the chamber of the main body and includes an inlet channel, an outlet channel and a particulate detector. The gas is introduced into the particulate measuring module through the inlet channel, and a concentration of particulates in the gas is measured by the particulate detector.