A gas detecting device includes a casing, an optical mechanism, a gas transporting actuator, a laser module, a particle detector and an external sensing module. The casing includes a chamber, an inlet, an outlet and a communication channel. The optical mechanism is disposed in the chamber. The optical mechanism includes an airflow channel and a light-beam channel. The airflow channel is in fluid communication with the at least one inlet and the outlet. The light-beam channel is in communication with the airflow channel. The gas transporting actuator is disposed on the optical mechanism. The laser module is disposed in the optical mechanism for emitting a light beam to the airflow channel. The particle detector detects sizes and a concentration of the suspended particles in the air. The external sensing module is installed in the communication channel to measure the air.