HVDC cable is the key equipment of HVDC flexible transmission technology, and the insulation thickness design of HVDC cable is based on the safety margin of lightning impulse voltage. In order to obtain the safety margin, test method of safety margin under lightning impulse voltage is proposed in this paper, and impulse waveform is calculated, simulated and verified. Impulse voltage waveforms with wavefront time (1~5) μs and half peak time (40~60) μs is obtained. An analysis method of lightning impulse voltage safety margin of dc cable is presented, and the electric field distribution of cable insulation is calculated. The research shows that the safety margin of HVDC cable can be obtained by using the test method proposed in this paper, the research results provide a basis for the design of HVDC cable structure.