The main topic of this paper is about design methodology. Firstly, itdiscusses the epistemology of design, summarizes the history of designmethod and concludes the trend of development. Secondly, it analyzes thecomplexity of design in depth and deems that design is an associated system,which includes the external determinative factors and the internalconstituent factors. Design means a combination of define the externalfactors and reorganize the internal factors. The uncertainty of externalfactors is the key point of design. This paper brings forward a newmethodology named Design Affairology. Affair is the overall external factors, which shape, determine and confinethe man- made objects. From affair, we can understand the relationshipbetween human and object, the context of time and space, the motivation,intention, feeling, and value of human. All of these define the object what itshould be. Affairology is to establish the object system from the process ofusers' behavior through researching the requirements of different people underdifferent environments and conditions ----"facts"; then organize the internalfactors such as principle, material, craft, form and color ---"seeking truth ".Design, seemingly is to create object, actually is story telling, emotionexpressing and reason explaining. The main character of Affairology is to emphasize the relationship, notelements; the whole, not the parts; the process, not the status. Aiming at aproblem, Affairology is a way to research the whole external factors of design,combine social science knowledge to understand user. It is not a rigid method,but a scientific way to produce knowledge of design. Directed by Affairologymethodology, designers should go to practice, to create method, to accumulateexperience and acquire knowledge. This paper is based on multi-discipline research. From the view of design,it integrates the knowledge of philosophy, economics, sociology,anthropology etc. It provides a valuable way for design method.