The invention relates to duplex fiber used for resisting cracking of concrete throughout the course. The duplex fiber is prepared with the following synthetic fiber by weight: 15 to 50 percent of a high strength and high modulus synthetic fiber, 20 to 40 percent of a medium strength and medium modulus synthetic fiber, 20 to 45 percent of a low strength and low modulus synthetic fiber. The duplex fiber is prepared with fibers of different length, different elastic modulus and different strength of extension through compounding, thus forming a low strength and low modulus, medium strength and medium modulus, high strength and high modulus compound system. In this way, the duplex fiber not only has the advantages of various individual filaments but also has the superimposed effect of compounding of various fibers. Accordingly, the duplex fiber has synergistic actions in different periods when concrete is coagulated and hardened, which not only can prevent the plastic cracking of concrete, but also can effectively control the cracks during the hardening of concrete. In addition, the duplex fiber can effectively improve the toughness and the malleability of the concrete so that the durability of the concrete is significantly improved.