Abstract: In the process of testing axisymmetric workpiece based on the digital radiographic system, caused by several factors, the center image is more bright than the margins. Some trend is produced. However, the flaw need to be distinguished superimposes on the trend, which increase the flaw testing difficulty. A extracting method for trend of signal based on empirical mode decomposition(EMD) was investigated. Through the analyse and experiment, it proved that the method could extract the trend of images more precisely and enhance the recognition rate of flaw comparing with the traditional trend analysis methods(such as least square approximation).