The invention relates to an ultrasonic time-domain detection method for a pore defect of an epoxy casting insulation part. The method comprises steps as follows: firstly, an ultrasonic velocity in a to-be-detected insulation material is detected; ultrasonic pulse is emitted to a to-be-detected insulation part, a reflected wave is detected and converted into a time-voltage signal, an abrupt change point of a waveform is found, time corresponding to the abrupt change point is recorded, the abrupt change point of the waveform corresponds to the pore defect in the insulation material, and the position and the dimension of the pore defect are calculated according to the time corresponding to the abrupt change point and the ultrasonic velocity obtained in the former step. With the adoption of the method for detection, the application of ultrasonic detection in a material such as the epoxy insulation part with large ultrasonic attenuation coefficient becomes possible. Besides, according to establishment of a detection system for implementing the method, the required device cost is lower, and radiation hazard caused by a radiographic detection method is avoided.