Abstract: The natural vibration performance of the over hanging beam for the launch vehicle of strap-on boosters is investigated using the subspace iteration procedure of ALGOR general program for the finite element method.Two kinds of calculation models are studied,one hinge the upper joints of boosters to the core vehicle and fix the lower joints to it,and the other fix both the upper joints and the lower ones of boosters to the core vehicle.Experiment result shows that the method of this paper is efficient.