The invention discloses a ventilation height adjusting device for internal maintenance of a gas or oil tank. The device comprises a core rod, a sleeve and locking bolts, wherein a core rod cone and a sleeve cone are arranged on the outer side of the core rod and the outer side of the wall of the sleeve respectively; the device is provided with a synchronous adjusting device; the synchronous adjusting device is provided with upper clamping arms and lower clamping arms, which can be butted with cone surfaces of the cones in a matching way; and a left screw and a right screw are driven by synchronous motors to realize lifting motion of the upper clamping arms and the low clamping arms. Through the device, an exhaust fan is fixed, and stable operation of the exhaust fan is guaranteed; and the device realizes free height adjustment to be suitable for gas tanks in different sizes, and realizes integration of air supply and air discharge to facilitate field operation.