The invention discloses a method and a device for extracting lithium, which is used for preparing lithium carbonate, from a lapidolite ore by a chloridizing roasting method. The method comprises the following steps of: firstly, mixing the lapidolite ore, calcium chloride and sodium hydroxide with a compound bonding agent for pellet fabrication; secondly, performing chloridizing roasting in a square-frame shaped track type roasting furnace; thirdly, leaching out soot dust by using solution containing sodium carbonate and potassium carbonate to ensure that potassium, sodium, rubidium and cesium enter the solution and convert the lithium into lithium carbonate; fourthly, filtering the mixture to obtain a lithium carbonate solid, and circularly using the filtrated mother liquor to leach out the soot dust; fifthly, when an alkali metal salt is close to be saturated, indirectly heating the filtrated mother liquor by using the residual heat of the gas in the roasting furnace to evaporate part of water; sixthly, passing CO2 into the filtrated mother liquor to perform carbonation; and seventhly, performing cooling crystallization to separate out a mixed salt of the sodium carbonate and the potassium carbonate, returning part of the mixed salt which is used as an auxiliary material mixed and roasted with lapidolite for cyclic utilization, using another part of the mixed salt as a carbonate reagent needed in dissolution, and using the rest part of the mixed salt as byproducts of the sodium carbonate and the potassium carbonate. The method has the advantages of high lithium recovery rate, good material comprehensive utilization, large equipment productivity, high production efficiency, small water consumption in the process and less wastewater discharge.