The invention belongs to the field of dielectric materials and discloses a flexible polyimide-based composite dielectric film material and a preparation method thereof. The flexible polyimide-based composite dielectric film material is prepared by stirring MXene and polyamic acid formed by dianhydride and diamine at room temperature to obtain a mixed solution of MXene/PAA, coating a base materialwith the mixed solution of MXene/PAA, and amidating at 150-550 DEG C, wherein the MXene is Mn+1Xn, X is selected from C or N, M is selected from Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo AND Mn transition metal elements. The preparation method of the flexible polyimide-based composite dielectric material is simple, the thickness is uniform and controllable, and the dielectric property is good; and as a supercapacitor device or an electromagnetic wave absorbing material, the flexible polyimide-based composite dielectric material has the characteristics of higher electromagnetic absorption efficiency, good cycle stability, small maximum reflection loss, wide absorption frequency range and the like.