Active pulsed thermography is adopted to test the inner defects of ceramic insulators, two high energy flash lamps were used to instantly heat the front surface of the tested ceramic insulator, whose temperature increases because of absorbing heat, the heat conducts into the ceramic insulator, thus it causes the temperature decrease of the front surface, the computer was used to control the infrared camera to capture and record this temperature decreasing process. Pulsed thermography was successfully used to detect impurities and holes in ceramic insulator based on the research of one-dimensional heat conduction model, the deduction and simulation of the first-order derivative of relative thermal contrast, and the processing and analysis of experimental temperature decreasing curves. The study also analyzed and discussed the sequential difference of the first-order derivative thermographic images. The results indicated that pulsed thermography could be used in the detection of inner defects of ceramic insulators.