FIR optical filter using birefringent crystals consists of N cascaded crystal waveplates with the same thickness between an input and an output polarizer.The variables determined by synthesis procedure are the angles of the optic axes of the crystal waveplates and the angle of the output polarizer.A Jones matrix based step-down iteration synthesis procedure for filter of this kind is proposed.Jones matrix describing such filters is the product of Jones matrixs of waveplates and polarizers used in filter.A simple step-down iteration relation and the orthogonal frequency response correspond to frequency response of filter is determined by this condition.The experimental result agrees with that provided by the reference and shows that the synthesis procedure is simple.