The invention discloses a liquid level volatile gas detection device for a water body with the frequently-changed water level. The liquid level volatile gas detection device comprises floating plates and gas collection chambers with the open lower portions, a collection chamber sealing cover is arranged on the upper portions of the gas collection chambers in a matched mode, a lifting support comprises a base, a lifting arm, a lifting rope and a lifting motor, and a liquid level height sensing device is arranged in each floating plate and comprises a liquid level sensor, a wireless signal transceiver and a battery; the liquid level volatile gas detection device further comprises two symmetrically-arranged site clearing structures, a site clearing motor device and an intelligent control device, the intelligent control device comprises a controller, and the controller is connected with the lifting motor and a regulating motor separately and controls operation of the lifting motor and the regulating motor. The liquid level volatilization gas detection device has the advantages that the internal pressure and the external pressure of the gas collection chambers are balanced, the detection precision is high, the detected water surface can be cleaned, the intelligence degree is high, the water surface height can be detected, and all-weather continuous automatic detection can be achieved.