The invention relates to an end sealing device of an annular air duct, comprising an annular air duct end sealing body and a floating supporting mechanism used for supporting the annular air duct end sealing body, wherein the floating supporting mechanism comprises a base fixed at the bottom of an annular liquid tank and an up-down floating mechanism arranged in the base; the annular air duct endsealing body is a hollow sealing body arranged outside the base and is arranged in a door-shaped sealing device through the floating supporting mechanism; the radial dimension and the transverse dimension of the top of the annular air duct end sealing body are respectively larger than the radial dimension and the transverse dimension of a through hole of a door-shaped sealing device cover plate; the annular air duct end sealing body is provided with a small hole on a side plate close to one side of a cooling area; and when the door-shaped sealing device cover plate moves to the upper side of the annular air duct end sealing body, the annular air duct end sealing body is in contact with the door-shaped sealing device cover plate, a door-shaped air duct inner annular plate and a door-shapedair duct outer annular plate and is subjected to relative motions. The invention provides the annular air duct end sealing device used for guaranteeing good sealing of the annular air duct end.