The flat bottom holes of different sizes and depths in C/SiC composite samples were nondestructively tested by using infrared pulse thermography testing method.The principle,processing and results of infrared pulse thermography testing were analyzed.The research results show that:for the same defect,the change of defect size with time approximately obeys the chi-square distribution,and when infrared pulse spread to the depth of defect,the shown defect size is biggest;Testing and distinguishing ability for small and deep defects can be improved after differential processing;the method can test defects of 2 mm diameter,and can not test defects of deeper than 4 mm depth(not bigger than 15 mm diameter) in C/SiC composite;The minimal diameter/depth ratio of tested C/SiC composite is 1.3.