Photomultiplier Tubes (PMT) are mostly used in air Cherenkov light detection in extensive air
shower of cosmic rays. The Cherenkov telescopes work at clear and moonless nights, therefore it
is susceptible to the background of stars in the sky; and with the Earth’s rotation and revolution,
the affection of background is different yet. In this case, we conducted an experiment to evaluate
the impact of night-sky background (NSB) to the gain of PMTs. By optimizing the voltage divider
of PMT, one can decrease the influence brought by NSB on the gain of PMT.