Abstract: The detection technology of radioactive hot area was studied by using in-situ HPGe detector with lead collimation. The key parameters of upper boundary, the thickness and the cover depth for radioactive hot area were determined by experiment and theory of this measurement model. In the process, the upper boundary of the radioactive hot area was confirmed by scanning measurement at first. Then the attenuation layer thickness of fixed point in the upper boundary of the radioactive hot area was confirmed. The thickness of the hot area was identified by alignment measurement and inclined measurement after the attenuation layer was determined. The technical method has obtained good results both in theory and experiment, and they are in good agreement.