Abstract:A one-dimensional wave equation with unbounded, when not considering the external force, can be directly solved by D' Alembert formula, When considering the external force, D' Alembert formula can not be used directly, but it can be solved by the superposition principle,and then solved.the special solution of the infinite string of forced vibration equation is transformed into the free vibration equation of infinite string, and then by the Darren Bell formula, fast results