The single-particle Green’s function for a dc-biased superlattices with single impurity potential varying harmonically with time has been obtained in the framework of u (t,t’) method and Floquet-Green’s function.The calculation of the local density of states shows that new states will emerge between the resonant Wannier-Stark states as a result of the intervention of time-dependent impurity potential,and the increase in electric field strength of impurity will result in the growing of the number of new states between the gaps of neighbouring Stark ladders.