The invention provides an expanding method of a sealing cover in a microwave digestion instrument. The expanding method comprises the following steps: (a) a sealing bowl of the sealing cover is moisturized, and is filled with water; (b) an expander is inserted in the sealing bowl with the water; (c) the sealing cover with the expander is put in an oven to prevent the water in the sealing bowl from flowing out, is baked by 6-8 hours at 98-102 DEG C, and is taken out from the oven for cooling by at least 18 hours; and (d) the expander is taken down from the sealing cover. The expanding method of the sealing cover preferably realizes shaping of the sealing cover, and can reduce dangers of the sealing cover to experience staff in the use process due to air leakage and splashing.