Abstract: In order to determine the evapotranspiration of plants easily and effectively, and provide easy-used apparatus which had high precision and good stability for studying on water use, the automatic monitoring system of large weighing lysimeter (LG-Ⅰ) was designed. The system adopts self-developed high-precision spring, high precision cantilever beam (SQC-A500 kg) sensor, verticle lift bracket and computer software-controlled processing program. The experimental results showed that the precision, stability and repeating ability of the apparatus were all better compared with the similar lysimeter used in China. Its maximum absolute error was from -25 g to 25 g, which was equivalent to 0.012 mm column. Its maximum relatively error was 0.7%. The apparatus could meet with the applying requirement. The data of the system can reflect evapotranspiration variation in a short time, so it is a sort of instrument which can monitor evapotranspiration of potted plant conveniently.