The invention discloses an epoxy leather tanning agent and a preparation method thereof. The preparation method comprises the steps that a part of epoxy chloropropane is dropwise added into isophoronediamine at first and reacts at the temperature of 25-35 DEG C for 2-4 h, after polyatomic alcohol and a catalytic agent are sequentially added, the other part of the epoxy chloropropane is dropwise added, after all the epoxy chloropropane is added, the temperature is raised to 50-70 DEG C, reaction is conducted for 3-8 h, an alkali compound is dropwise added when the temperature is lowered to 20-45 DEG C, reaction is conducted for 2-4 h, finally, a neutralizer is added for neutralization till the pH value is neutral, an upper-layer product is rotated and filtered, the epoxy leather tanning agent comprising the following formula is obtained, and the formula can be seen in the specification. The epoxy leather tanning agent is low in production cost, can be stably stored for longer than 6 months at the normal temperature, and is low in viscosity, good in heat stability, wide in molecular weight distribution and short in tanning time, the tanned leather is pure white, full and good in hydrothermal stability and yellowing resistance, and the storage stability is good.