The invention belongs to the technical field of automatic detection, and relates to a continuous measurement device for the relative change amount of the outer diameter of a pipeline. The continuous measurement device comprises a mechanical measurement system, a signal processing and transmission system and a computer, and is characterized in that two long and thin hollow rollers with the front end being in the shape of a circular cone tip are respectively installed on cantilever shafts thereof through bearings, the two cantilever shafts are respectively fixed on sliding blocks thereof, two parallel cylinders serve as common sliding rails of the two sliding blocks, the two sliding blocks can perform opening and closing movements along the sliding rails at the state that the two rollers arekept to be parallel and are perpendicular to the sliding rails, the detected pipeline is located between the two rollers, springs arranged on the sliding rails in a sleeved manner ensure a displacement sensor fixed between the two sliding blocks to acquire a change value of the outer diameter of the pipeline in the measuring process. The continuous measurement device can perform high-precision continuous measurement on the change amount of the outer diameter of the pipeline, the detected data is transmitted to the computer at a high speed and can be played back subsequently, the whole processis carried out automatically, missing inspection is avoided, the efficiency and the accuracy are high, and accurate, efficient and real-time detection can be performed on positions where a crawler can reach.