Abstract: Instrument has become one of the key technologies for promoting the development of science and economy on the ground that instrument plays a very important role in collecting, monitoring, managing and controlling information. A weir is often used for measuring the runoff yield of a basin in hydrology process and soil erosion. In this study, in order to auto-monitor the sediment concentration in runoff flow, a self-designed instrument was introduced which was used for auto-sampling water from the storm runoff in a small catchment. The reliability and applicability of the instrument was validated by the simulation experiment, and the average relative error was only 0.42%. In addition, compared with traditional water sampling instruments, the self-designed instrument has a higher level of automation, is more convenient and suitable for auto-sampling water from runoff in fieldwork with unattended operation. Therefore, the self-designed instrument for auto-sampling water from storm runoff has a good application prospect.