Abstract: Pipeline drift diameter detector of electronic DN - 700 is able to prove a change degree in pipeline diameter automatically and in quantity on-stream, distinguish pipeline' valve, bend and girth weld and know its corresponding mileage location. Through on-site application, it reaches design property requirement and conforms to actual desire. The extended rubber cup in diameter detecting arm with mechanical bevel-shaped connecting rod contacted closely with pipe wall improves measuring sensitivity and the application of electronic computer technology makes data's acquisition, treatment and analysis more accurate and solves a technical difficulty in pipeline diameter detection which can't be solved for many years at home.