The melt blending theory of PET/PBT alloy is discussed, which includes compatibility, crystallization and transesterification during the melt blending process of the two materials. The melt blending system of PET/PBT is miscible in the amorphous area, and the producing process has great influence on its crystallinity. Because of the synergistic effect on the crystallization process of PET and PBT blending，adding nucleation system can promote crystallization during moulding and processing phases while the transesterification decreases the probability of crystallization. The mechanical properties of the alloy and their influencing factors are researched. The hardness of the alloy is affected by the molecular weight and the cooling rate when quenching, its flexural strength can be improved through filling inorganic filler. The alloy has strong notch sensitivity and small notched impact strength, therefore impact modification is always considered the most important research subject. As a kind of engineering plastics, the PET/PBT alloy is mainly applied in the automobile area at present, and has good prospects in the applications in electronics, mechanical industry and automobile parts.