an adjustable locking device includes a main body , at least a link structure , and at least a buckling device . the main body includes a connection portion . the connection portion provides one end of the link structure to connect thereon . the link structure includes a link element and an adjusting element connected to the link element . the link element provides a buckling device to pivotally connect thereon . the buckling device includes an actuating element and a buckling element . the actuating element and the buckling element are mutually connected , whereby the link structure can be moved within the connection portion of the main body , so as to change the position of the buckling device . the connection tightness between the main body and the connection port is thus enhanced .