The invention relates to a preparation method of a novel leather greasing agent. The preparation method aims at achieving the purposes that the process is simple, convenient and feasible, the three-waste emission load is low, environment friendliness is achieved, product quality is stable, and the greasing performance is excellent. According to the technical scheme, the preparation method of the novel leather greasing agent includes the following steps that 1, continuously-input animal and plant grease is heated to a certain temperature, then gas-phase SO3 is input for an SO3 membrane type sulfonation reaction, and the SO3 combining amount is controlled; 2, chlorine is introduced in the sulfonated animal and plant grease, chloridized animal and plant grease sulphonate is prepared through a chlorination reaction, and the chlorine content is controlled; and 3, the chloridized animal and plant grease sulphonate is neutralized through alkali and decolored, and then the leather greasing agent product is obtained.