The invention discloses a detection method for internal defects of a composite insulator. The method comprises the steps that a composite insulator sample is equivalent to a black box, when internal defect information such as internal pores, internal impurities, mandril cracks and mandril and sheath debonding of the black box cannot be directly obtained through the appearance, high-frequency pulse voltage signals are applied to the composite insulator sample through an electrode connected with the composite insulator sample, detection signals for corresponding defects of the composite insulator sample are obtained through detection instruments such as an oscilloscope, and waveform parameters and an image database of the detection signals are obtained by analyzing and processing the detection signals. Corresponding relations between the detection signals and the typical defect types are established and serve as reference standards for judging detection results.