The invention relates to a gamma-ray source-based ray nondestructive testing device for a foreign body inside a small-diameter metal tube. The device comprises a gamma-ray source, a protective cover, an array detector, a signal converter, a fixed support, a gamma-ray controller and an image processing system, wherein the gamma-ray source is a Se-75 radiation source; the array detector is a scintillator array which is composed of a strontiumiodide array, an optical element array and an optical filter arranged between the strontiumiodide array and the optical element array; the gamma-rays are emitted by the gamma-ray source, pass through a pipeline to be detected and are received by the array detector; the signal convertor converts light signals into electronic signals and transmits the electronic signals to the image processing system; and finally the foreign body is displayed. The device of the invention can conveniently, quickly and accurately detect the foreign body inside the small-diameter metal tube; by adopting a real-time display technology, various inconveniences brought by the departed shooting technology can be avoided; and the device is based on the computer image processing technology, therefore the images can be intuitively displayed and the data can be far passed and stored.