The invention discloses a method for preventing reducing of the surface strength of a sand mold. The method comprises the steps that a sand mold cavity is manufactured in a sand box according to the conventional process, a riser channel is formed between the sand mold cavity and the exterior, an outlet hole is formed in the top of the riser channel, a base plate is arranged on the periphery of theoutlet hole, and a fan is placed on the base plate and supplied with electricity by an electricity supply; and a fan is started while molding sand is blown in through carbon dioxide, and wind of thefan is led into the sand mold cavity through the riser channel. The fan is a large-power industrial fan. A rubber pad is arranged on the lower portion of the base plate. According to the method for preventing reducing of the surface strength of the sand mold, certain air pressure is applied to the sand mold cavity through the action of the wind blown by the industrial fan, and in this way, the carbon dioxide blown in secondarily cannot permeate into the surface of the sand mold under the action of the air pressure but permeating into the non-mold-cavity surface, so that the phenomenon of overblowing of the carbon dioxide to the surface of the mold cavity is avoided, the strength of the sand mold is effectively ensured, and accordingly, the quality of a cast product is ensured.