Abstract: With the popularization and application of aeronautical electromagnetic metamaterials in aircraft radome, the problem of lightning protection needs to be solved urgently. In this paper, the impulse voltage generator, high-speed video and ordinary camera were used to test the U_50% impulse voltage and discharge path of the block-shaped electromagnetic metamaterial plate, and the combined gap and surface discharge characteristics of different grounding modes and voltage polarities are studied. The experimental results show that the surface discharge morphology of the block-shaped metamaterials is not affected by the polarity of the voltages, which shows an irregular broken line discharge path, and the main discharge channel is not unique. There are secondary discharge channels flowing to another grounding position. More secondary discharge channels are generated when it is closer to the discharge attachment point, and the whole surface discharge morphology is distributed in a network. There are many main discharge paths along the surface under lightning impulse voltage. Before combined gap breakdown, the surface of electromagnetic metamaterial will produce self-discharge path along the surface, which is one of the final main discharge paths along the surface. The lightning impulse discharge characteristics of the development process of combined gap are mainly related to the voltage polarity. Under the positive impulse voltage, the downward streamer is generated by the high voltage rod electrode, and there is only weak corona discharge on the surface of electromagnetic metamaterial.Under the negative impulse voltage, there are opposite upward and downward streamers, respectively. Under lightning impulse voltage, lots of metal microstructure unit arrays on the surface of electromagnetic metamaterials will produce up-streamers and surface self-discharge paths, which is easy to lead the main lightning discharge channel to the metamaterial radome, resulting in the shielding failure of splitter bar.