The traditional document-based design method has some challenges in the design process of manned spacecraft avionics system, such as ambiguity in information transmission, difficulties in tracing requirement changes, and the inability to quickly verify in the early stage of design. To address these challenges, the application of the model-based systems engineering method (MBSE) in the design of manned spacecraft avionics systems is studied. Based on the "V" development process, requirements analysis, functional analysis, system design, and design verification are conducted. The closed loop from requirements to design to verification is realized using the bilingual modeling systems SysML and Modelica, and multi-attribute joint decision-making techniques are introduced to finish the multi-scheme architecture trade-offs. The results show that the MBSE method can effectively ensure system design consistency and requirement change traceability, as well as reduce the risk of early design errors, which provides a reference for the further application of digitization design in spacecraft avionics systems. Finally, suggestions are made for the future use of MBSE.