The invention relates to a method for preparing an environmentally friendly interior wall decorative plate. The method includes mixing PVC resin, PET resin and an inorganic filler; kneading the mixture in a kneading machine; adding dioctyl phthalate, magnesium stearate and a foamer into the kneading machine; kneading the mixture; adding the mixture into a mold of a heating plate of a foaming machine; raising the temperature to 174-175 DEG C; maintaining the pressure; then cooling to 55-58 DEG C; disassembling the mold for pressure relief; then putting a foam plate into a drying oven having a temperature of 65-70 DEG C; taking the foam plate out of the drying oven after 2 h; cooling the foam plate to room temperature; then putting the foam plate into a drying oven having a temperature of 48DEG C; performing thermal insulation for 10 h; and taking the product out and cutting the product to obtain the decorative plate. The decorative plate is prepared by adding a large quality of treatedinorganic raw materials into the resin and performing foaming treatment, and a foaming process does not need other adhesives, and therefore, the thermally insulating plate does not generate formaldehyde or other harmful substances when in use, and can be used indoor reliably so that material environment protection performance is improved.