The invention relates to a beam lifter. The beam lifter structurally comprises a main beam, a car frame, a suspension assembly, a fixing supporting leg, a main beam, an amplitude variation supporting leg, a hoisting mechanism and a hydraulic system; the lower portion of the fixing supporting leg is in rigid connection with the car frame and the suspension assembly on the corresponding side, the upper portion of the fixing supporting leg is in rigid connection with the main beam, the lower portion of the amplitude variation supporting leg is in rigid connection with the car frame and the suspension assembly on the corresponding side, the upper portion of the amplitude variation supporting leg is in flexible connection with the main beam, and the hoisting mechanism is arranged on the main beam and can move left and right along the main beam. The fixing supporting leg and/or the amplitude variation supporting leg have/has the heightening function, active and automatic adjustment of the height of the supporting leg on one side or supporting legs on the two sides can be achieved, the main beam keeps horizontal all the time, the beam lifter can well adapt to the work environment with the large the ground height difference, and stability in the heightening process can be ensured.