GIS equipment in service is compact in layout and complex in structure. The shooting angle of the X -ray is limited due to such constrains as space

height and testing tools. Such non-vertical irradiation situations as center deviation of beam and detector flipping often occur

and the X-ray testing spectrum shows such deformation as translation and stretching

resulting in significant measurement errors in dimension of GIS component and difficulty in accurately identifying some defects. In view of above typical issues

the geometric models for vertical irradiation such as X-ray beam center offset

detector flipping

and non-vertical irradiation such as X-ray beam center offset and detector flipping are set up. The influence of X-ray beam center offset and detector flipping angle on the size of X-ray detection spectra is studied

the calibration formula for X-ray detection spectra under different non vertical irradiation condition is proposed

a calibration method for defect size measurement in the operating GIS using X-ray detection under non-vertical irradiation condition is developed and achieving measurement and calibration of defect size under full transmission conditions is achieved. The relevant research results have achieved good application results in the diagnosis and characterization of defects in ultra-high voltage GIS disconnector at site.