In Iran, due to shortage of water resources, the application of closed hydroponic systems is one of the priorities of research on the development of greenhouse cultivation. In these conditions, there is a strong need for reuse of nutritional solutions due to the high economic costs and environmental problems. Replacing the base nutrient solution in a constant manner at each growth stage causes a nutritional imbalance in closed hydroponic system. In this experiment, this technique was evaluated to determine the most suitable concentration of substitutional solutions in a closed hydroponic system for greenhouse cucumber. The experiment was conducted in a completely randomized design with three replications and four treatments. The experimental treatments contained fixed solution (control) and application of the base solution and replacement of the nutrient solution adjusted in different stages of growth by assuming dry weight ratio (kg) to transpiration (liters) in three levels (1/200, 1/300 and 1/400). Substitutional solution of dry weight to transpiration ratio of 1/300 improved the leaf nutrient concentration of cucumber and had the maximum yield per plant and net photosynthesis rate. The highest efficiency of photosynthetic water use was related to the solution of dry weight to transpiration ratio of 1/200.