The invention discloses a weathertight butterfly valve. The weathertight butterfly valve comprises a cylindrical housing, a valve plate, rainwater circulation pipelines and a swirling switch, wherein rainwater collecting holes are formed in the plate surface of the valve plate, the valve plate is located in the housing, and the shape of the valve plate is matched with the shape of an inner chamber of the housing; each of the rainwater circulation pipelines is provided with a water inlet and a first water outlet, and is mounted on the housing in a penetrating manner, the water inlets are located in the housing, the first water outlets are located outside the housing, the inner ends of the rainwater circulation pipelines are connected with the valve plate, and the rainwater collecting holes communicate with the water inlets; and the swirling switch is mounted at the outer ends of the rainwater circulation pipelines. The swirling switch is rotated to drive the rainwater circulation pipelines to rotate, and the valve plate is linked with the rainwater circulation pipelines, so that the inner chamber of the housing is opened or closed by the valve plate; and the inner chamber of the valve plate is closed by the valve plate, and rainwater falling on the valve plate in the housing is discharged out of the housing along the rainwater circulation pipelines under the action of gravity. According to the weathertight butterfly valve provided by the invention, the use efficiency of a ventilation system can be improved, the operating workload of a platform can be reduced, and the ventilation system of the platform can be restored as soon as possible.