  The water Cherenkov detector array (WCDA) for the large high altitude air
shower observatory(LHAASO) will employ more than 3600 hemisphere 8 inch
photomultiplier tubes (PMT). The good time performance of PMT, especially the
transit time spread (TTS), is required for WCDA. TTS is usually defined as the
TTS of single photoelectron, and usually determined by using single
photoelectron counting technique. A method using the photoelectron spectrum is
researched for the measurement of TTS. The method is appropriate for
multi-photoelectrons and makes it possible to measure the TTS of different
photoelectrons at the same time. The TTS of different photoelectrons is
measured for Hamamatsu R5912 with the divider circuit designed in specifically.
The TTS of single photoelectron is determined to 3.3 ns and satisfies the
requirement of WCDA.
