Abstract-According to the purpose of energy saving and low cost, a set of optimization design method is put forward for designing a forced circulation solar hot water system which can meet the need for all hot water loads. A digital model of solar water heating system is constructed in which the relationship between guarantee rates of solar energy and total system costs is considered. And multi-objective optimization model for the system is established. Using MATLAB genetic algorithm toolbox the objective model is optimized and solved. Pareto no dominated solution sets for multi objective function are analyzed, and the optimum area of its collector was obtained.