A biomagnetic microsphere and a preparation method and a method for protein isolation and purification therefor. The outer surface of a magnetic microsphere body of the biomagnetic microsphere has at least one liner polymer with a branched chain; one end of the linear polymer with a branched chain is covalently coupled to the outer surface of the magnetic microsphere body, and other parts are free on the outer surface of the magnetic microsphere body; a backbone of the linear polymer is a polyolefin backbone, and no cross-linking agent is required in the backbone forming process of the linear polymer. The prepared biomagnetic microsphere can implement efficient elution of target proteins and effectively reduce the retention time and retention ratio of the target proteins, and it is easy to operate and widely used.