Abstract: Satellite remote sensing technology is able to support large-scale, all-weather and all-time dynamic monitoring and plays an important role in water management. However, there are still no special satellite constellations designed for the water sector. The spatial and temporal resolutions of satellite images are not enough to meet the requirement of the water sector and cannot effectively support the construction of intelligent water sector and digital twin. This paper proposed the conception of a small satellite constellation for the water sector—a SAR/ optical hybrid constellation with shorter than 1-hour temporal resolution and less than 1-meter spatial resolution, and described the overall concept and key technologies. The constellation can provide customized and professional services for water sector and related agriculture, environmental protection, and other sectors to support the major national water strategy.