The blue-green synergy is an effect of complementary and mutual promotion in structure and function between hydrology and green space, matching size and distribution. The blue-green synergy is an important breakthrough to solve the problems of urban drought and flood, water use imbalance, and habitat degradation. The concept of blue-green synergy degree is proposed, and its evaluation model and indicator system are established. A mapping method for the blue-green synergy degree is proposed. Taking the rural-urban fringes of Xi’an city as the object, the optimization scheme of the green space pattern was generated based on the evaluation of the blue-green synergy degree. The results show that the blue-green synergy degree increased to 0.30 in areas with lower blue-green synergy degrees after optimization, which reached a moderate level, proving the feasibility of the optimization method. At the same time, the optimization method is applied to design the core planning area of Xi’an high-speed railway east station, realizing the organic combination of blue-green problem governance and urban landscape construction. The research can provide a new idea for the study of the synergistic effect of hydrology and green space and the development of the blue-green spatial planning and ecological construction in the complex urban and rural environment. This research can also provide a reference for optimizing and improving urban green space planning methods.