The invention provides a single main gap of a spark gap. The main gap comprises a shell and a gap system in the shell, wherein the gap system comprises a flashover gap and an afterflow gap; and an intermediate electrode is arranged in the flashover gap and divides the flashover gap into two flashover gaps. The two gaps are connected in series to facilitate improving the superiority of the operational reliability of the main gap, and the difficult problem caused when the single main gap is used in the situation of smaller series compensation degree or lower rated voltage is solved, so that the working reliability and stability of the single main gap are greatly improved.