An adjustment device for the steering assembly of an electric scooter is provided. The adjustment device is at least composed of a body, two confining plates, an elastic element, a cam wheel member, two bearing rods, and a sliding shaft. The sliding shaft has its bottom end pin-joined to a joining seat of the electric scooter. The rest of the sliding shaft passes through the two confining plates and the elastic element positioned between the two confining plates, all of which are installed inside the body. The body in turn is pin-joined to a main shaft of the steering assembly. The wheel cam member has a wheel cam installed between the two confining plates and pin-joined to the body. The two bearing rods are inserted into the body from the top and bottom of the body respectively and function as pivoting points to the two confining plates. By operating the wheel cam member to release the confinement of the two confining plates to the sliding shaft, the sliding shaft could slide freely through the body of the adjustment device and thereby offer various degrees of inclination for the steering assembly.