The invention discloses an experimental device for outdoor measuring the underflow exchanging amount and an implementing method thereof. A foundation pit is excavated at the bottom of a riverbed, water filtering type geotextile is laid at the bottom of the foundation pit, a rectangular container formed by splicing a plurality of plastic plates with hole boxes is arranged on the geotextile, every two adjacent plastic plates with the hole boxes are fixed through a detachable clamping plate and bases with clamping grooves, and each plastic plate with the hole boxes is provided with a plurality of rows and lines of evenly-formed circular holes; grit stacked to be wavy is contained inside the rectangular container, and a plastic net is laid on the grit; one ends of columnar sensors penetrate through the circular holes of the plastic plates with the hole boxes in the middle of the rectangular container and are buried into the grit, the other ends of the columnar sensors are fixedly arranged on a PVC plastic plate through screws, and the detachable plastic plates with the hole boxes are arranged on the side walls of the periphery of the rectangular container besides the portion in contact with the PVC plastic plate. The experimental device is high in efficiency and convenient to operate, the underflow exchanging amount of an actual river can be measured, and data is more accurate.