How to coordinate and trade-off limited water and land resources to improve water resource utilization efficiency, increase food production capacity, protect ecosystems and enhance carbon sequestration remains a complex scientific issue. Taking the Sanjiang Plain as a case study area, water resources, crop planting structure and ecosystem distribution under conventional irrigation and water-saving irrigation scenarios are spatially coordinated and optimized through a distributed hydrological model (CWatM) and an improved a multi-objective optimization model with GridLandOpt method. The results showed that: (1) Water resources in the Sanjiang Plain fluctuated between 15.67 and 47.92 billion m3 during the 2000–2019. (2) Compared with that before optimization, under the conventional irrigation mode, irrigation water use was reduced by 2.3 billion m3, total carbon sequestration increased by about 12.53 %, economic benefits decreased by 3.22 %. The planting structure of rice, corn and soybeans needs to be adjusted from the original 3:2:1–2.2:2.4:1.4. Wetland area increased by 2.34 %, ecological benefits increased by 12.53 %, and ecological connectivity improved by 2.79 %. (3) Compared with that before optimization, under the water-saving irrigation mode, irrigation water was reduced by 4.5 billion m3, total carbon sequestration increased by about 11.21 %. The planting structure of rice, corn and soybeans needs to be adjusted to 2.3:2.4:1.3. Wetland area increased by 0.81 %, ecological benefits increased by 0.80 %, and ecological connectivity improved by 2.30 %. The research results can provide technical support for the sustainable use of water resources, stable grain production and improvement of ecosystem service functions in China's major grain-producing areas.