The invention discloses a leveling and deepening fixing agent and a preparation method thereof. The leather leveling and deepening fixing agent is a dyeing assistant with the functions of leveling, deepening and fixing colors, and an assistant molecule comprises the groups of -NR2, -NH2, -NHR, -COONa, and the like and is a compound with an amphoteric structure. Under an alkaline condition, the molecule carries negative charge and has a leveling function for dyeing acid dye, the auxochromic groups of -NR2, -NH2, -NHR, and the like on the assistant molecule act on the dye to deepen the color andplay a deepening function. Under an acidic condition, the groups of -NR2, -NH2, -NHR, and the like have positive charge and play a fixing function for the acid dye. The assistant is a multifunctionalproduct, can play the functions of leveling, deepening and fixing colors for dying leather and can simplify a leather preparing technology.