Abstract: The diameter-variable oil and gas pipeline detector is a kind of pipeline detection device that can simultaneously conduct in-service detection on pipelines with different diameters within the interval of a pig launching and receiving station.The advantages lie in its strong passing capacity, wide application and cost saving.Through the research on the structural design and sealing performance of the diameter-variable cup for diameter-variable detection, a set of 40-48 in(1 in=25.4 cm) diameter-variable cup structures suitable for oil and gas media were proposed.The rationality and feasibility of the structural scheme of the diameter-variable cup structure were verified by finite element simulation analysis, combined with the methods of diameter-variable pipeline detector traction and gas sealing driving test.The actual industrial application on site shows that the structural design and sealing performance of the diameter-variable cup meet the design index and the field's application requirements.The research has laid a solid technical foundation, for breaking the long-term monopoly of foreign countries on diameter-variable pigging and detection technology, and for the early realization of serialized industrial field application of selfdeveloped diameter-variable pigging and detection device in China.