The invention discloses a high-temperature stabilizing agent, belonging to the field of admixtures for cement slurry used for cementation of thick-oil thermal recovery wells. The main objective of theinvention is to overcome the problems that conventional silica sand stabilizing agents cannot totally meet the requirements of thick-oil thermal recovery well exploitation on the strength of cement sheaths and that the later strength of the cement sheaths greatly declines. The high-temperature stabilizing agent mainly comprises, by mass, 74.75% of a strength reinforcing agent, 20.57% of a reinforcing adjuvant and 4.68% of an activity activator. The high-temperature stabilizing agent allows set cement for cell cementation to have high early-stage compressive strength and excellent capability in resisting high-temperature strength decline, can effectively improve inter-layer sealing capacity of cement sheaths and prevent the channeling of oil, gas and water, and is of critical significanceto improvement of thick-oil thermal recovery well cementation quality, enhancement of thick-oil exploitation and prolongation of oil well life.