Water is the primary limiting factor for plant growth and crop production in the loess hilly-gully area of the Loess Plateau.Because the annual precipitation is relatively low and its seasonal distribution is variable in the area,it is extremely important to study the soil water balance such as storage water and adjusting plant water requirement.Lysimeter is an excellent and classical instrument for measuring each component of field water balance equation such as evapo-transpiration and drainage.It is wildly used in field evapo-transpiration,rainfall infiltration experiment,and experimental hydrology.In 2005,the weighting sensor system and power source were improved and host computer and data colleting software of Lysimeter were updated,which were installed in the Ansai Research Station of CAS for studying filed water evaporation and transpiration transformation.After improvement,the instrument can work under Windows interface and the data collecting is more convenient.The modified Lysimeter shows some of the new characteristics,which are:the weight of the contained soil column is 19~26 t,the weighting precision is 152 g(0.05 mm) and can measure evapo-transpiration and groundwater replenishment and leakage;the measured area and soil depth are 3 m²(1.5 m×2 m) and 3.0 m,respectively,which may fully display water transformation in the crop rooting system such as root system development,water consumption,soil water and groundwater transformation,and groundwater level dynamic;the supply and drainage system can simulate the change of groundwater level,which can reflect the actual conditions and allow researches on soil water dynamic during crop growth.