Under the condition of deficient irrigation, based on the field water balance model and crop yield calculation model, a multi-objective optimization model was put forward to optimize irrigation date and corresponding quantity of irrigation simultaneously. In this model, irrigation date and the quantity of irrigation water were regarded as optimization variables, and maximization of relative yield of crops and minimization of total quantity of irrigation water during whole growth stage were as the optimization objectives. In addition, in the respect of solving the model, a new genetic chromosome structure was designed for this kind of optimization problems. On the basis of non-dominated sorting genetic algorithm (NSGA-Ⅱ), an improved genetic algorithm based on grouping sort was put forward to solve the optimization model. The model was verified by irrigation schedule optimization of corn. Optimization result showed that relative yield of crops increased by 0.07~0.20 when the total irrigation water quantity was same, and the irrigation water quantity decreased by 7~15 cm when the same relative yield in comparison with the calculation results of literature ［9］.