The invention discloses a water-soluble polyurethane curing agent and a preparation method thereof. The curing agent comprises toluene diisocyanate, trimethylolpropane, polyethylene glycol, polycarbonate diols, acetic ether and a catalyst. The preparation method comprises the following steps: adding 18-25% of toluene diisocyanate, 20.01-80% of acetic ether and 0.01-40% of polycarbonate diols, adding 1-5% of trimethylopropane for reaction in three batches, adding 0.01-40% of polyethylene glycol for reaction in two batches, and then keeping the temperature at 58-63 DEG C for 3 hours; and adding0.1-0.5% of catalyst, reacting for 2.5-4 hours while the temperature is kept at 50-60 DEG C, cooling until the temperature is reduced to 20-30 DEG C, and discharging. According to the invention, the preparation method is simple, and the curing agent prepared by utilizing the method can react with water-soluble acrylic resins or water-soluble alkyd resins to form a film when being mixed with the water-soluble acrylic resins and water-soluble alkyd resins. Thus, the curing agent is not only applicable to curing at normal temperature but also applicable to preparation of high temperature curing paints.