The utility model relates to a quantitative watering valve for a tobacco seedling transplanter. The tobacco seedling transplanter, similar to a wind closing device of a flour machine, comprises a casing which is provided with a water inlet, a water outlet and an air outlet; the casing is internally provided with a turntable water bucket, and each water bucket of the turntable water bucket is internally provided with a water quantity regulating block; and two seal rings are arranged between the turntable water bucket and the casing. The valve is arranged on a shaft of a front wheel disc of the transplanter, the turntable water bucket is connected with the front wheel disc of the transplanter, and the turntable water bucket is driven by the front wheel disc for rotating together. When each water bucket of the turntable water bucket rotates above and an opening of the turntable water bucket is communicated with the water inlet, water injects the water bucket, and the air outlet exhausts; and when each water bucket of the turntable water bucket continuously rotates below, an opening of the water bucket is communicated with the water outlet of the casing, water is poured into a duck mouth of the transplanter positioned right under the opening of water bucket so as to complete one-time watering process, and the watering quantity can be rapidly adjusted by increasing and reducing the water quantity in the water bucket. The valve has the advantages of having a simple structure, being adjustable in water yield, and capable of adapting to the transplanter for watering before transplantation.