In order to study the performance of MRPC, which will be used for the upgrade of the Endcap TOF in
BES III, a T0 system is composed of high speed response PMTs H6533 coupling with BC420 plastic
scintillators. Because the T0 system should offer a toughly strict timing start, the high precision
electronics based on VME system were used to test the SPS (single-photoelectron spectrum) of H6533
PMT. A suitable operation voltage for optimal performance is confirmed by researching the
dependence relationship between the PMT gain and energy resolution of SPS (single-photoelectron
spectrum). At the end, the timing resolution of this type of T0 system was 41.6 ps in the cosmic ray test
and 39.1 ps in the proto beam test.