Abstract: An expert system for nutrient solution management of hydroponic culture of tomato and cucumber was developed. The design object, the basic structure and functions of the system were described. The composition of knowledge database was introduced in detail. The system can accomplish model-based reasoning. The models quantitated dynamic absorption regulation of nitrogen (N), phosphorus (P), potassium (K) and calcium (Ca) for tomato and cucumber at different development and growth stages, as well as different environment. Decision supporting for concentrations of principal ionic species, pH value, EC value, temperature and concentration of dissolved oxygen in nutrient solution can be realized, especially under adverse circumstances (e.g. above 32℃ and below 5℃ of air temperature, under 3000 lx of sunshine, etc.). Based on an expert system framework, nutrient solution management expert system for other vegetable crops can be developed.