Abstract: Compare the applicable scopes, targets and conditions of 3 typical standards which give measuring methods for focal spot sizes of X-ray tube(IEC 60336, EN 12543 and ASTM E1165). Following a detailed analysis of method principle, testing condition and result process, it focused on analyzing the similarities and differences of contents described in 3 standards by pinhole imaging, and pointed out the differences of methods, applicable scopes and targets. Conclusions of this paper can be used by users as good references and basis of knowing roundly about the measuring methods for focal spot sizes, of choosing appropriate methods according to different intentions and requirements.