The invention discloses a live detection device applied to a lightning arrester with a discharge gap and a control method, and relates to the technical field of lightning arrester monitoring. According to the live detection device applied to the lightning arrester with the discharge gap and the control method, accurate time service triggering is carried out through the Beidou time service module circuit, total current data of the ABC three-phase lightning arrester are synchronously collected, accurate time service triggering is carried out through the Beidou time service module circuit again, the digital power amplifier circuit is started, and the discharge gap is detected. High-frequency alternating-current voltage is injected from the discharge gap for multiple times, the ground potential of the lightning arrester is raised, disturbance is applied to capacitive current, data acquisition is carried out on total current, Fourier transform is carried out on multiple groups of total current data, and the total current is acquired; lightning arrester operation data such as full voltage, full current, resistive current, third harmonic, fifth harmonic, seventh harmonic, power and phase angle difference of the ABC three-phase lightning arrester are obtained, and full-state live detection of the lightning arrester with the discharge gap is achieved.