With the continuous development of aerospace technology, the complexity and reliability requirements of onboard software are increasing day by day. In order to effectively test and verify the onboard software, this paper proposes a graphical simulation case design system of onboard software based on reflection technology. Through the graphical interface, this system enables users to visually construct simulation scenarios and design cases, and realizes high flexibility and scalability by using reflection technology. This paper details the architecture, key technologies and application examples of the system, and verifies its effectiveness and superiority in the test of onboard software.