China is one of the few countries which suffer worst natural disasters in the world and it is an urgent need for effective monitoring methods. Small satellite constellation for environment and disaster monitoring and forecasting is the first small-satellite constellation dedicated to environment and disaster monitoring and forecasting of China. Disaster risk assessment and warning system for environmental and disaster monitoring and forecasting is one of the core systems of small satellite constellation ground system, it undertakes the task of disaster risk analysis and evaluation. Based on the Unified Modeling Language (UML) technology, this paper defines the system's function requirement and the non-function requirement and the business concepts and their relationships of system through the requirement analysis and domain modeling, and refines the system architecture design through the logical structure design and physical architecture design. The practice proved that architecture design based on UML is very useful for specifying the business objectives, business functions and constraints limit in disaster risk assessment and warning system and guiding the late technical development work.