A cationic cyanuric chloride derivative tanning agent and a preparation method thereof are disclosed. The method includes: mixing 9.22-36.88 parts of cyanuric chloride, 21.35-100.31 parts of a solvent, and 7.12-33.44 parts of deionized water, all by mass, in an ice-water bath to obtain a mixture A; at a temperature of 0-5° C., adding 2.98-20.44 parts of a tertiary ammonia compound to the mixture A, and subjecting the resulting mixture to a reaction for 4-6 hours, during which its pH value is adjusted with an acid-binding agent solution to 6.0-7.0, to obtain a mixture B; filtering the mixture B, washing the filter cake, and vacuum drying for 4-6 h to obtain a solid C, and grinding to obtain the cationic cyanuric chloride derivative tanning agent.