The present invention relates to a polyolefin which exhibits a narrow molecular weight distribution, high strength, and a high stretching ratio, and a preparation method therefor. According to the present invention, it is possible to provide polyethylene with high catalyst activity, high molecular weight, and controlled molecular weight distribution by using, in polymerization of an olefin monomer, a hybrid supported metallocene catalyst in which a borate compound is used as a second cocatalyst. In addition, if the polyolefin is processed, it is possible to produce a fiber with excellent mechanical properties and an excellent stretching ratio.