A PDAbased diffusion simulating system of weather modification is introduced briefly, and the system design and technical nnovative points of the system are described. The system focuses on the conversion of the findings in theoretical researches on catalyst diffusion into computeraided applied technology and can simulate the diffusion process of cloud seeding through the fast computeraided visualization processing to provide support for the design of weather modification operation schemes, being simple and easy to use. Considering the needs of weather modification operations in the system analysis and design, the system has the rational data structure, software architecture, and visualized and friendly manmachine interaction interface.