This paper provides a forward design process of avionics interface definition for civil aircraft using integrated modular avionics, which is from functional interface definition to physical interface definition. This paper first defines the interface category and expounds the corresponding relationship between the avionics interface design process and the development stages of civil aircraft in the whole life cycle. For functional interface definition, an interface design method using modeling tool Capella was proposed. For avionics interface definition(including hardware and software definition, port definition, transmitted data definition and etc.), the design procedure, process and method were provided. At the end of the paper, combined with the forward design method of the avionics interface definition, a specific case was given for engineering implementation reference.