Abstract: Based on the Shandong triggering lightning experiment, the overvoltage measurement on the external floating roof oil tank model stroke directly by triggered lightning was carried out. The current at the bottom of lightning channel and the voltage at different positions of the oil tank were measured synchronously during one triggered lightning with four return strokes (RSs) on July 12, 2021. The results show that the lightning contains four negative RS. The maximum negative peak current is −34.01 kA and the rise time from 10% to 90% is 0.11~19 μs during the four RSs. The amplitude and change trend of voltage waveform with time are basically same at the tank top and floating disc and the voltage pulse duration is about 4 μs. The potential difference waveform between them presents three troughs and four peaks, and the period is about 1 μs and the amplitude ranges from −21.3 kV to +18.8 kV. As a whole, the higher the lightning current intensity is, the shorter the 10%~90% rise time will be, the larger the peak voltage and their potential difference of the tank top and floating disc will be, and the stronger the amplitude will be. Calculation reveals that, when lightning strikes the oil tank directly, if there is no reliable connection between the floating disc and the oil tank, the potential difference between them is most likely to ignite the oil and gas in the tank. Finally, some protection suggestions are put forward.