The invention discloses a wellhead sealing device which comprises a diameter-variable flange body (12), wherein the upper end head and the lower end head are fixedly connected with a mandrel hanging type casing head and an air injection wellhead four-way respectively through a thread-carried bolt II and a thread-carried bolt I, the wellhead sealing device further comprises a casing body (11), a casing coupling (1), an annular steel plate (2) and a sealing ring (6) which are integrated to the position of the same axis of the diameter-variable flange body (12). The casing coupling (1) and the casing body (11) are in threaded connection, the annular steel plate (2) is of two semicircular combined structures, is combined and assembled to a lower end face of the casing body (11) in a sleeving mode and arranged at the position tightly matched with the lower end face of the casing coupling (1), and the sealing ring (6) is arranged on the casing coupling (1) in a sleeving mode. The wellhead sealing device has the advantages of being reasonable in structural design, low in cost, high in safety coefficient and convenient to use, is long in service life, simple in manufacturing process, safe and reliable and is used for an air injection well.