[en] Photomultiplier Tubes (PMT) are commonly used in air Cherenkov light detection in extensive air shower of cosmic rays. The Cherenkov telescopes usually 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 impacts of night-sky background (NSB) to the gain of PMTs. By optimizing the voltage divider of PMT, the influence brought by NSB on the gain of PMT can be decreased.