The invention discloses a waste foundry sand reutilizing method. The waste foundry sand reutilizing method includes the following steps that (1) waste foundry sand is crushed through a crushing mill to obtain waste sand particles of 40-100 meshes; (2) the waste sand particles obtained in the step (1), flyash and cement are mixed according to the weight ratio of 2:1:2 to obtain a mixture, and a water repellent and water glass are added into the mixture to obtain a mixed material; (3) the mixed material is put in a mold, and a semi-finished product is obtained through a molding press; and (4) the surface of the semi-finished product is cleaned, a vehicle door is subjected to flame treatment so that the surface adsorption capacity of the vehicle door can be enhanced, and spraying of outer-layer paint is performed. By means of the waste foundry sand reutilizing method, the waste foundry sand can be effectively recycled, and pollution is reduced.