This paper proposes and designs an open modular onboard avionics system to improve the standardization and generalization design capability as well as the intelligent and networked application level, which has passed the engineering application verification. The avionics system is built on the basis of hierarchical bus architecture and generalized hardware modules and software components to form a set of standard service function packages, which can be selected and installed according to different mission scenarios, significantly enhancing the shelf integration and assembly development capability of the avionics system, and at the same time, with the help of unified information and communication service network and hardware and software base components, a solid foundation is laid for modular upgrade and expansion of avionics system, device level functional reconfiguration and reorganization, and system level fault tolerance and recovery capabilities, and a strong technical support is provided for the overall high-quality, high-efficiency and effective development and operation of the spacecraft.