Provided is a process for maintaining cleanness and running of a cavity during microwave cracking of waste tires, comprising the following steps: A, preparation before running: to meet the reaction conditions of cracking, before starting running, charging medium gas into a cracking cavity at normal pressure; B, cleanness and running during microwave cracking: after starting running, conveying a material into a microwave cracking cavity by a material conveying device in a horizontal direction, and performing microwave cracking on the material at the lower part of the microwave cracking cavity; in addition, introducing the medium gas from an air inlet passage formed at the upper part of the microwave cracking cavity, causing the introduced medium gas to move toward the lower part of the microwave cracking cavity, driving cracking mixed gas generated by cracking to pass through the material, and discharging the gas out of the cracking cavity from an air outlet passage formed at the bottom part of the cracking cavity. The process has the advantages that condensable cracking oil steam and other pollution gases generated by cracking are effectively discharged, the pollutions in a cracking device are reduced to the greatest extent, and the cleanness and effective running of the device are ensured.