The invention relates to the field of optical measurement, a device for estimating the absolute light responsivity of a photomultiplier tube under low light intensity conditions, mainly comprising a laser, an attenuator I, a dark box, a beam splitter I, an attenuator II, a plane mirror I, a shutter I, Shutter II, plane mirror II, beam splitter II, cassette, silicon photodiode, photomultiplier tube to be tested, said beam splitter I, attenuator II, plane mirror I, shutter I, shutter II, plane mirror II, beam splitter II , cassette, silicon photodiode, and photomultiplier tube to be measured are all located in the dark box, and the silicon photodiode and photomultiplier tube to be measured are located in the cassette, composed of laser, attenuator I, beam splitter I, shutter I, plane mirror II, Beam splitter II and silicon photodiode form optical path I, which consists of laser, attenuator I, beam splitter I, attenuator II, plane mirror I, shutter II, beam splitter II, and silicon photodiode. I. The attenuator II has a certain angle with the optical axis of the measurement system.