The invention discloses a forming method for a microwave oven cavity or an oven cavity. The forming method comprises the following steps of preparing sheet materials; performing surface treatment on the sheet materials; performing welding and shaping, and powder ceramic coating spraying. According to the forming method, the rough sand finish surfaces of the sheet materials are prepared firstly; then a welding and forming way is adopted, and next, the ceramic coating is sprayed, so that it is ensured that the cavity is not deformed, and the ceramic coating which is low in energy consumption, high in strength, resistant to high temperature, non-viscous and easy to clean can be obtained; the sheet materials are subjected to sand blasting treatment firstly and then the powder ceramic coating is sprayed, so that the conditions of high energy consumption, high-temperature solidification and deformation, or toxic fluorine-containing gas decomposition at a high temperature of the conventional enamel coating or Teflon coating can be avoided; and in addition, the cavity forming method disclosed by the invention achieves the beneficial effects of simple process, easy operation, high use safety, and strong high-temperature resistance.