The purpose of this paper is to investigate the dynamic behavior of the bridge subjected to the traveling horizontal transverse loads due to nosing and yawing of the multiple vehicle of Maglev. It were found that the perturbation equation of motion of the thin-walled beams under traveling horizontal transverse follower load systems becomes Hill's equation and that parametrically excited unstable coupled vibrations occur. The boundary frequency equations of the simple parametric resonance, from which the instability regions are estimated, are obtained by Bolotin's method. The instability regions of a simply supported beam are illustrated, taking into account the effects of load mass.