The invention relates to an adjustable expansion mandrel which comprises an adjustable mandrel, an ejection nut, an expansion sleeve and a bolt. A conical face is arranged at one end of the adjustable mandrel and is sleeved with the expansion sleeve fixed to the adjustable mandrel through the bolt, a gasket is arranged between the expansion sleeve and the bolt, the ejection nut fixed to the adjustable mandrel is arranged on the other side of the expansion sleeve, a semi-circular key is further arranged on the inner side of the expansion sleeve, and a key groove matched with the semi-circular key is formed in the conical face of the adjustable mandrel. The adjustable expansion mandrel has the advantages of being compact in structure, reliable in positioning, convenient to dismount, good in practicability and the like.