The invention discloses PBT-PET (Polybutylece Terephthalate-Polyethylene Terephthalate) composite elastic fibers and a preparation method of the PBT-PET composite elastic fibers. A polybutylene terephthalate and polyethylene terephthalate composite material is fused and then is compounded and spun into a spring shape, and a cross section thereof is in a round structure, wherein a PET component is distributed at the outer side of a spiral and a PBT component is distributed at the inner side of the spiral; the content ratio of the PET to the PBT is (3 to 7):(7 to 3). The PBT-PET composite elastic fibers have the following beneficial effects that (1) the PBT is used for replacing original PTT (Polytrimethylene Terephthalate) and PET composite spinning and a monofilament breaking problem of T-400 fibers in a warp-wise spinning process is solved; (2) the fibers have the advantages of high rebound resilience, good stability, soft fabric, good drapability, flat cloth surface, strong crease resistance, moisture adsorption and quick drying, smooth hand feeling, stable size, no need of ironing and easiness of being cared; (3) the PBT-PET composite elastic fibers can be used for independently and purely weaving and also can be interwoven with cotton, sticky fibers, dacron, chinlon and the like. Textiles of the PBT-PET composite elastic fibers can be widely applied to fabrics of trousers and skirts, sports wear, stockings, jean fabrics, underwear, top-grade woman dresses and the like.