In order to use the smallest inductor to improve the band rejection of BAW ladder filter, an optimizing methods for inductor which regulate the band rejection characteristic of BAW laddertype filter are proposed in this paper, the methods include two sequential optimizations for the value of inductor. Firstly, the principle of improving the band rejection of BAW ladder filter by the inductor is analyzed, the formula of static capacitance, and the formula of electrical input impedance of FBAR with a series inductor, the conclusion of “the improvement results of band rejection is larger and the value of inductor is smaller when the inductor series in shunt branch with a larger FBAR resonator area than in the other branches” is achieved, which determines the best position of inductor’s, followed by which the determined value of inductor is smaller. On this basis, by degrading the excellent value of band rejection approximately equal to the specification of band rejection, the value of inductor will be reduced further. Through two sequential optimizations for the value of inductor，the position and the smallest value of inductor will be determined rapidly. Lastly, a 31/2-stage BAW ladder filter is used to verify the validity of the optimizing method.