The invention relates to a clean method for extracting vanadium during roasting vanadium slag. The method comprises the following steps that vanadium in the vanadium slag is converted into calcium vanadate which is stable at high temperature by adopting a calcification roasting method, high-concentration ammonium sulfate and/or ammonium bisulfate solution are utilized for leaching roasted clinkerto enable most of ammonium metavanadate to be crystallized and then to be mixed with tailings to form crystal-containing slag, then the crystal-containing slag is dissolved in a hot liquid to obtain avanadium-containing solution, and efficient extraction of the vanadium can be realized. According to the method, the problem of high-temperature vanadium volatilization does not exist, the comprehensive recovery rate of the vanadium is higher, ammonia gas is not generated in the leaching process, and compared with other easily-decomposed ammonium salts, the conversion rate of the vanadium is higher under the same concentration and the same leaching temperature; and meanwhile, the problems of small solubility of ammonium metavanadate in the ammonium salt solution and low vanadium extraction rate can be effectively solved. In the subsequent processes, ammonia gas generated by calcining ammonium metavanadate is adsorbed by using a sulfuric acid solution, the ammonium sulfate can be resynthesized, the ammonia recycling can be realized, the whole process is clean, the production cost is greatly reduced, and the method has good economic benefit and application prospect.