The utility model discloses a draining device for a crop water culture system. The draining device comprises a draining pipe and a plurality of solution collection pipes communicated with the draining pipe. The draining pipe comprises a first horizontal section and an arch-shaped section, wherein the lowest point of the pipe wall in the arch crown of the arch-shaped section is not lower than the highest point of the pipe wall in the first horizontal section. The arch-shaped section is arranged at one end of the draining pipe, and therefore the first horizontal section can be filled with nutrient solutions, and the solution collection pipes can drain all the nutrient solutions in a water culture bucket in a siphoning mode. The problem that residues of original nutrient solutions influence a new nutrient solution system in the process of replacing the original nutrient solutions with new nutrient solutions can be avoided. When the draining flow at the tail end of the draining pipe is larger than the draining amount of the nutrient solutions in the water culture bucket, negative pressure can be formed in the first horizontal section in front of the arch-shaped section according to the siphoning principle, a siphoning effect on draining of the nutrient solutions in the water culture bucket can be achieved, and the draining speed of the nutrient solutions in the water culture bucket can be increased.