The invention provides a cable insulation state diagnosing method and diagnosing apparatus. The method comprises the following steps of exerting a periodically changing index waveform voltage on a detected cable; calculating the medium loss tangent value curve of the detected insulating cable according to the frequency domain phase relationship of the waveform voltage and a current on an index wave rising edge; exciting partial discharge signals of the detected insulating cable through the waveform voltage on an index wave falling edge; acquiring and recording the partial discharge signals and analyzing partial discharge characteristic parameters of partial discharge defects. The equipment with a small volume is lighter. Excitation source equipment has a small volume. No charge accumulation effects exist. Less cable damage is realized. The excitation voltage grade is adjustable. The method can be used for detecting the medium spectrum of high-voltage distribution cables. Partial discharge information of samples can be directly acquired for partial discharge positioning and pattern recognition.