The invention relates to the field of a high molecular material, and particularly relates to a brominated polyethylene/ultra-high molecular weight polyethylene fiber composite material and its preparation method. The brominated polyethylene is periodic brominated polyethylene; the brominated polyethylene/ultra-high molecular weight polyethylene fiber composite material applies an ultra-high molecular weight polyethylene fiber composite material induced crystallization method to adjust the aggregation structure of the brominated polyethylene under the condition of not changing the chemical structure of the brominated polyethylene, thereby realizing the regulation of the brominated polyethylene crystal structure; the brominated polyethylene crystal structure is regulated from a general triclinic system to an orthorhombic crystal, thereby improving the melting temperature and the physical performance; the method is simple and easy to obtain.