The invention discloses a leather tanning agent and a preparation method thereof, and belongs to the technical field of leather machining. The leather tanning agent comprises, by weight, 30-40 parts of polyetherdiolbranchedacrylicresin, 15-20 parts of modified grapheneoxide, 3-5 parts of aluminum citrate, 1-3 parts of sodium hexametaphosphate and 50-60 parts of water. The leather tanning agent is combined by adopting specific components, especially polyetherdiolbranchedacrylicresin, flexiblebranchedpolyetherdiol is led into polyacrylic acid to improve the selecting and filling property of acrylicresin, the sense of water and elasticity of leather are improved, then polyetherdiolbranchedacrylicresin, an aluminium tanning agent, modified grapheneoxide and the like are matched and used, the leather softness is improved, meanwhile, the leather grain flat is fine and smooth, the leather contraction temperature can be improved, the leather tanning agent is suitable for tanning of various leather, and the good application prospect is achieved.