Dynamic stability of flexible beam under travelling horizontal follower load systems is investigated by applying the fundamental equation, governing the dynamic elastic stability, which is derived by using the linearized finite displacement theory. It is shown that the fundamental equation of the flexible beam under the above travelling load systems becomes Hill's equation and parametrically excited unstable coupled vibration occurs. Furthermore, the boundary frequency equations of simple parametric resonance are obtained by Bolotin's method and the stability maps of a simply supported beam are shown, as influenced by load mass and damping. Lastly, the behaviors of vibration of the beam in the vicinity of boundary of a main untable region are examined by numerical integration.