Abstract: Abstract: In city green space planning, plant evapotranspiration (ET) and rainwater supply haven't considered. In this study, city water-saving green space system was planned based on GIS technology by considering the plant ET and rainwater supply. We would take full advantage of rainwater resources and plan ecological green space system with water-saving function for the core area of Songbei in Harbin. The GIS technology was used for extraction of underlying subsurface of collectable rainwater for calculation of utilizable rainwater. Plant ET was calculated by Penman-Monteith formula. The results showed that the rainfall in the study area was concentrated in a range of 485-671 mm. The average rainfall of 30 years was 524.5 mm. The rainfall maximized in the year of 1994 (524.5 mm) and minimized in the year of 1989(345.5 mm). The rainfall from June to August accounted for 66.4% of the annual rainfall. Among 5 levels, light and moderate rain were the main levels of rainfall and total of them accounted for 69.3% of the total rainfall. Rainwater resources utilization was annually 5 126 800 m3. The total of the plant ET was annually 6 397 100 m3 from May to October while in the same time the total amount of rainwater could be collected 4 561 700 m3. The rainwater resource could reach 71.31% of plant ET. Water demand of plants was mostly consistent with rainfall period. The rainwater harvesting amount in July and August could fully meet the requirement of water by plants, however, the rainwater was less than the required water by plants in the other months especially in May. The rainwater harvested in May only accounted for 28.35% of plant ET. The distribution map of water-saving green space of study area was built up. The green space with the slope between 3° and 15° was designed as the infiltration type greenbelt, and the slope less than 3° was as the water-collecting green space, and by the DEM model, the area of potential water-saving green space could reach 1 664.19 hm2, which was 37.10% of the research area. By using the theory of hydraulic engineering and geographic information science, and taking the water-saving green space as the main body, the water-saving green space system with using rainwater resource was planned for the study area. The system structure was one green ring, three green belts, three vertical and three horizontal green belts, one green isolation belt, one green core, one green region, two green spaces and multiple green points. Five basic types of green space were planned for the study area with park green space (19%), production green space (2%), protective green space (8%), affiliated green space (10%) and the other green space (4%). So the green coverage rate could reach 48% and park coverage rate would reach more than 85%, and the long-term green space rate would reach 43%. This paper showed a method for planning urban green space system with water-saving ecological function so that the water resources in urban could be fully utilized.