The invention relates to a magnetic collagen modified cationic flocculant and a preparation method thereof. The magnetic collagen modified cationic flocculant is mainly prepared by combining a magnetic fluid with waste tanning collagen which is taken as a raw material. The magnetic collagen modified cationic flocculant has the beneficial effects that a magnetic material is applied to preparation of the flocculant, and a solid and liquid separation effect of a flocculation process is enhanced by the effect of a magnetic field, and thus the quality of water discharged after the flocculation process is improved, the water treatment running cost is lowered, and water content of floccules is reduced. Resource utilization of tanning wastes is realized, and pollutant emission caused by tanning is reduced. A magnetic collagen hydrolysis product is subjected to graft copolymerization and cationic modification, and a modified product has good flocculent properties. The magnetic collagen modified cationic flocculant is easy to degrade by microorganisms in environment, short in existence time in the environment and low in residual quantity.