In designing a three-channel non-imaging passive ranging system based on oxygen absorption, determining the absolute spectral sensitivity of the photomultiplier tubes is of great significance to ensure the ranging accuracy of the system. In order to overcome the difficulty of monochromatic light power measurement caused by high sensitivity and saturation of photomultiplier tube, we propose a measurement method of absolute spectral sensitivity of photomultiplier tubes. In this method, a system light source with adjustable flux is constructed using an integrating sphere and an adjustable slit, and a standard detector with known spectral sensitivity is used to measure the radiant power of the monochromatic light source. An absolute spectral sensitivity measurement system for photomultiplier tube is established, and the absolute spectral sensitivity of three H10722-01 photomultiplier tubes is measured with this method. In order to test the effectiveness of the proposed measurement method, a short-range passive ranging experiment is conducted in external field, the measured absolute spectral sensitivity of the photomultiplier tubes is used as the spectral sensitivity parameter of the passive ranging system. The experimental results show that in the range of 60-300 m, the average relative error of distance measurement is 6.25%. The experimental results prove the validity of this absolute spectral sensitivity measurement method.