The present invention discloses a self-flowing farmland underground drenching solution collecting device which belongs to the field for researching the environment science, agrology and plant nutriology. The device of the invention comprises an eluviation disk, a water-discharging pipe, a liquid-collecting pipe, a breathing pipe, a liquid drawing pipe, a sampling bottle, a buffering bottle and a vacuum pump; the eluviation disk is used for collecting the drenching solution; the liquid-collecting pipe is connected to the water outlet of the eluviation disk through the water-discharging pipe, the drenching solution in the eluviation disc can automatically flow into the liquid collecting tube; and the vacuum pump is used for extracting the drenching solution collected by the liquid-collecting pipe to the sampling bottle through the liquid drawing pipe under the buffering of the buffering bottle. Comparing with the prior art the invention has the characters of reasonable structure, convenient manufacturing, easy actualizing and the like, and the invention is especially suitable for monitoring the influence of the soil solutes of nitrogen, phosphor, agriculture chemicals and the like to the groundwater.