The invention provides an automatic quantitative seed supply device for a precision seeder. The device comprises a controller, stepping motor, a seed supply workbin, a first seed sliding board, a second seed sliding board, a third seed sliding board, a first sponge seed protecting block, a driving belt wheel, a driven belt wheel, a conveyor belt, a seed falling conveying trough, and a second sponge seed protecting block. The controller is electrically connected with the stepping motor; the driving belt wheel is connected with an output shaft of the stepping motor, the conveyor belt is arranged on the driving belt wheel and the driven belt wheel, and the first sponge seed protecting block and the second sponge seed protecting block are respectively arranged above the two ends of the conveyor belt; the seed falling conveying trough is fixed on the bottom of the seed supply workbin; and the first seed sliding board, the second seed sliding board and the third seed sliding board are fixed on the side wall of the seed supply workbin. The invention can realize automatic quantitative accurate seed supply and has low degree of damage to seeds; one time of seed supply can satisfy the long-work time of a seeder; and the seed supply can be quantitatively set according to the actual need, thereby providing conditions for precision seeding.