Abstract: Since 1980s, as the advantage of modular construction technology in avoiding the labor shortage, high human cost and strict environmental requirements of the construction place, some large construction companies of petroleum and natural gas engineering in developed countries have adopted it in quantities of engineer constructions, and their design and construction level both developed rapidly. At present, the modular design technology of large petrochemical industry and natural gas treatment device has not been mastered by domestic design companies. The modular design technology and skids were adopted only in the fields such as oil field ground engineering, medium and small natural gas treatment devices and so on. The scientific and systematic technical system has not been developed. Based on the analysis presented above, the project group of "modular design and construction technology research" of CNPC, based on a project of 300×10⁸ m³/a natural gas treatment plant, has deeply studied the modular design technology, and clearly defined the concepts in the modular design process of petroleum and natural gas devices, including the skid, module, module/skid specification, modularization, module splice, pipe splice, ship loose and so on. The above concepts are rich in content, clear in extension and practical. It can meet the requirements of modular construction process. By analyzing above key concepts, it can help the engineering technicians to understand the basic concepts and principles of modular design, so as to guide them to the correct direction of modular design.