The invention discloses a gas expanding agent for cementing medium-temperature and high-temperature wells and application thereof, and belongs to the technical field of cement admixtures used in the field of cell cementing in an oilfield completion technology. The gas expanding agent comprises the following components in parts by weight: 20-30 parts of a gas generating agent, 5-10 parts of a gas generating aid, 0.5-1.0 part of a foam stabilizer and 40-90 parts of a filler, wherein the gas generating agent is at least one selected from N,N'-dinitrosopentamethyltetramine, azodicarbonamide, barium azodicarboxylate, 4,4'-oxybisbenzenesulfonyl hydrazide and p-toluenesulfonyl hydrazide; the gas generating aid is at least one selected from urea, sodium benzoate, zinc oxide, aluminum oxide and lead oxide. The gas expanding agent can meet requirements of on-site well cementing construction at the circulating temperature of 80-120 DEG C, effectively compensates the pressure loss and the volume shrinkage during cement stone solidification, improves the cement stone cementing quality and improves the anti-gas channeling capacity.