The invention relates to a method for preparing sodium bicarbonate in the ammonia-soda process, which takes part of heavy alkali separation mother liquor as the component of carbonized supply liquor to carry out carbonization reaction under 15-42 DEG C by a two-stage air inlet mode. The method is characterized in that the heavy alkali separation mother liquor undergoes ammonia absorption, coolingand solid sodium chloride supplementation to form ammonia salt mother liquor; the ammonia salt mother liquor and the stock solution are mixed into half mother liquor as the carbonized supply solutionaccording to a volume ratio of B=3-6; wherein, the total concentration TNH3=5.8-7.3mol/l, the total chlorine concentration Tcl=4.5-5.3mol/l, and the carbonization degree of the carbonization reactionsolution Rc=170-188 percent. The method is characterized by high conversion rate of sodium chloride which is as high as 80-84 percent, low consumption of equipment investment, raw materials and energyandsimple carbonization operation; meanwhile, the method can prepare the heavy alkali crystal with 120-150mum of average grain diameter and uses a centrifuge to separate heavy alkali crystal mush; moreparticularly, the method can avoid a good many disadvantages brought by cooling heavy alkali saturated solution.