The invention discloses an early-strength polycarboxylic acid water reducing agent, a preparation method therefor and an application of the early-strength polycarboxylic acid water reducing agent. The water reducing agent is prepared from the following raw materials in parts by weight: 40-48 parts of polymethacrylic acid, 12-20 parts of polyzinc sulfate silicate, 8-16 parts of polyacrylamide, 3-10 parts of magnesium carbonate and 16-24 parts of polyethylene polyamine. The preparation method comprises the steps of mixing polymethacrylic acid with a prepared polyethylene polyamine solution, carrying out heated stirring treatment, adding polyzinc sulfate silicate and magnesium carbonate into the solution, carrying out heated stirring treatment, then, adding polyacrylamide into the solution, carrying out heated stirring, and cooling the solution to room temperature, thereby obtaining the early-strength polycarboxylic acid water reducing agent. The early-strength polycarboxylic acid water reducing agent has the advantages of high water reducing rate, short setting time, low concrete gas content, high early strength and the like and can be used for remarkably improving the early strength of cement concrete when the water reducing agent is used for prefabricating the cement concrete, so that demolding is achieved in advance, the production efficiency is increased, and the energy consumption of production of the prefabricated cement concrete is lowered; the applicable temperature is relatively low, and the water reducing agent can be used under the condition that the temperature is above -5 DEG C; and the compatibility among all ingredients is relatively good, so that the water reducing agent can be uniform and stable in relatively-long-time storage.