The invention relates to the technical field of building materials and in particular to a waterproof reinforcing alkali-free liquid setting accelerator and a preparation method thereof. The setting accelerator comprises a modified aluminate solution, a stabilizer, a pH value adjusting agent, magnesium biphosphate and water, wherein the preparation method of the modified aluminate solution comprises the following steps: putting stearic acid into a reaction kettle, performing heating to 70-80 DEG C, performing heating to a molten state, powering on a stirring device, adding ammonium hydroxide toimplement a reaction, performing cooling to 35-50 DEG C, adding aluminum sulfate, aluminum isopropoxide and water into the reaction kettle, performing an ultrasonic reaction, performing cooling to anormal temperature, and further adding calcium hydroxide to implement a reaction, so as to obtain modified aluminate. The setting accelerator is applied to synthesis for preparing an alkali-free liquid setting accelerator, the prepared alkali-free liquid setting accelerator has a waterproof function, is capable of improving the durability of concrete, reducing the rebound rate of the concrete, improving the stability of a product and remarkably improving early-stage and later-stage strength of sprayed concrete, and has significant practical application values in concrete construction.