The invention discloses a preparation method for continuously synthesizing sodium amide in indigotin production process, which comprises the following steps: adding liquid sodium metal into an ammonia-sodium pre-synthesizing tower continuously at certain temperature; and circulating the liquid sodium metal by an ammonia-sodium circulating pump, wherein one part of the liquid sodium metal returns to the pre-synthesizing tower to pre-react with ammonia gas which is not completely reacted in an ammonia-sodium synthesizing tower at a temperature of between 300 and 400 DEG C and the end gas is discharged from the upper part of the tower; and the other part of the liquid sodium metal enters into the ammonia-sodium synthesizing tower to react with the ammonia gas at a temperature of between 300 and 400 DEG C to obtain the sodium amide. The preparation method improves the reaction yield and the product quality, shortens the reaction period, reduces the generation of pollutants, reduces the equipment investment and the staff labor intensity, improves the working environment, and greatly reduces the energy consumption.