The invention provides a variable diameter detector (4), comprising a base (41), a detection arm (42) and an angular sensor (43), wherein the detection arm (42) is composed of a arm part (421), a fixed part (422) and an installation part; the installation part is mounted between the arm part (421) and the fixed part (422) and is connected with the base (41) in a pivoted manner; and the angular sensor is used for detecting the rotary angle of the detection arm in relative to the pivot shaft of the base. The invention also provides a pipeline variable diameter detection device using the above variable diameter detector. The variable diameter detector can directly detect the rotary angle of the detection arm in relative to the pivot shaft of the base through the angular sensor without need ofmounting structures such as calculus meter, so that diameter change of pipelines can be directly calculated, thus having higher precision.