A pneumatic and hydraulic elevating seat tube assembly of a bicycle, the pneumatic and hydraulic elevating seat tube assembly comprising: an outer tube; a movable inner tube disposed in and axially movable relative to the outer tube; an elevating adjustment unit having a fixed inner tube, a fixed piston, a piston pin, an actuating shaft, a floating piston and an elastic isolator, the fixed inner tube being disposed in the movable inner tube, the fixed piston being disposed in the fixed inner tube, an upper oil chamber being formed between the fixed piston and a top end of the fixed inner tube, the fixed piston having a pin hole communicating with the upper oil chamber, the piston pin being inserted in and axially movable relative to the pin hole of the fixed piston, the actuating shaft being disposed in and axially movable relative to the movable inner tube and the outer tube and connected with the piston pin, the floating piston being disposed in the fixed inner tube, sleeved onto and axially movable relative to the actuating shaft and located between a bottom end of the movable inner tube and the fixed piston, a gas chamber being formed between the floating pistop and the bottom end of the movable inner tube, a lower oil chamber being formed between the floating piston and the fixed piston and communicating with the pin hole of the fixed piston, the elastic isolator being attached to the floating piston, the elastic isolator being provided at an end thereof with a first annular groove facing toward the lower oil chamber, the elastic isolator being provided at another end thereof with a second annular groove facing toward the gas chamber; and wherein the floating piston is provided with an axial hole and sleeved onto the actuating shaft by the axial hole; the elastic isolator is disposed in the axial hole of the floating piston and sleeved onto and axially movable relative to the actuating shaft; characterized in that: wherein the floating piston has an internal flange and a washer groove; the internal flange is extended from an internal wall of the axial hole toward the actuating shaft and separated from an external surface of the actuating shaft by a predetermined interval; the washer groove is concaved from the internal wall of the axial hole in a direction away from the actuating shaft and located below the internal flange; a top end of the elastic isolator is abutted on a bottom surface of the internal flange; a bottom end of the elastic isolator is abutted on a washer disposed in the washer groove. an outer tube; a movable inner tube disposed in and axially movable relative to the outer tube; an elevating adjustment unit having a fixed inner tube, a fixed piston, a piston pin, an actuating shaft, a floating piston and an elastic isolator, the fixed inner tube being disposed in the movable inner tube, the fixed piston being disposed in the fixed inner tube, an upper oil chamber being formed between the fixed piston and a top end of the fixed inner tube, the fixed piston having a pin hole communicating with the upper oil chamber, the piston pin being inserted in and axially movable relative to the pin hole of the fixed piston, the actuating shaft being disposed in and axially movable relative to the movable inner tube and the outer tube and connected with the piston pin, the floating piston being disposed in the fixed inner tube, sleeved onto and axially movable relative to the actuating shaft and located between a bottom end of the movable inner tube and the fixed piston, a gas chamber being formed between the floating pistop and the bottom end of the movable inner tube, a lower oil chamber being formed between the floating piston and the fixed piston and communicating with the pin hole of the fixed piston, the elastic isolator being attached to the floating piston, the elastic isolator being provided at an end thereof with a first annular groove facing toward the lower oil chamber, the elastic isolator being provided at another end thereof with a second annular groove facing toward the gas chamber; and wherein the floating piston is provided with an axial hole and sleeved onto the actuating shaft by the axial hole; the elastic isolator is disposed in the axial hole of the floating piston and sleeved onto and axially movable relative to the actuating shaft; characterized in that: wherein the floating piston has an internal flange and a washer groove; the internal flange is extended from an internal wall of the axial hole toward the actuating shaft and separated from an external surface of the actuating shaft by a predetermined interval; the washer groove is concaved from the internal wall of the axial hole in a direction away from the actuating shaft and located below the internal flange; a top end of the elastic isolator is abutted on a bottom surface of the internal flange; a bottom end of the elastic isolator is abutted on a washer disposed in the washer groove.