Abstract: TOFD (Time of Flight Diffraction) detection technology was carried on an in-service 5 000 m³ spherical tank, showing that it could effectively detect the internal defects such as pore, slag, incomplete fusion, crack. According to the testing results, the safety status level on spherical tank was determined and the causes of the defect were discussed. It is recommended that the ring welds of spherical tank need to be periodically inspected.