The utility model provides a device for testing the nutrient competition relation between plants of the same species on a water planting condition. The device is provided with a culture box, a hemispherical cover, a gas check valve, nursery trays and field planting cups. An upper port of the culture box is connected with an opening of the hemispherical cover in a sealed mode. The gas check valve is arranged on the wall of the hemispherical cover. The gas check valve is arranged to make gas flow into the culture box unidirectionally. The two field planting cups are connected with the bottom face in the culture box. The wall of each field planting cup is provided with at least one water inlet hole. The two nursery trays are placed on the two field planting cups respectively. Each nursery tray is a cylinder, wherein the upper portion of the cylinder is large, and the lower portion of the cylinder is small; the upper end face and the lower end face of the nursery tray are both open. The hemispherical cover is colorless and transparent. The field planting cups are non-transparent. The device can guarantee normal growing of hydroponic plants and accurately measure the nutrient content, absorbed by the hydroponic plants, of a pre-purified water body, the nutrient competition relation between unrelated plants can be determined easily and conveniently through a calculation formula according to the measured nutrient content of the plants, and then an advantageous plant disposition mode through which the water nutrient absorbing effect is good is selected and used for purifying the water body.