In order to solve the problem of low thermal extraction rate of lignite, the raw coal is pretreated with acid and alkali as solvent, and then the pretreated raw coal is subjected to thermal extraction reaction to prepare ultra-pure coal under the process conditions of operating temperature 380 ℃, normal pressure and coal dissolution ratio of 10∶1. The effects of different acid and alkali solvents on thermal extraction rate are investigated. The results show that compared with raw coal, the thermal extraction rate of lignite treated with different kinds of acid-base solvents has been improved to different degrees, and the thermal extraction rate of lignite pretreated with potassium hydroxide is the highest, reaching 66.5%. Infrared spectrum shows that pretreatment of raw coal with acid-base solvent can effectively destroy the molecular structure of coal, expose inorganic mineral salts and fully expose organic functional groups. Therefore, higher yield of ultra-pure coal can be obtained through pretreatment, and the usage of high-quality coal in coal blending and coking industry can be reduced.