Abstract: This paper describes the development of non-destruction test device by γ-ray scanning and solving the problem of non-destruction test in ultra-thick iron castings.firstly,the work pieces to be tested are divided into several regions.The device collects the regional data in order,and we can judge the exist of defect and the size of the defect after the analyst compared the abnormal dat（a Number of mutants: the value is bigger usually）and uniform numeral.Defects position of the ultra-thick iron casting is determined according to the corresponding number of abnormal data,and we have tested ultra-thick iron castings such as shielding door by the device.The error of test results is within ±5 mm.