Abstract: In this paper the time performance of a plastic scintillator detector with an area of 10 cm×10 cm and read out by multiple silicon photomultipliers (SiPMs) at both ends was studied. The results tested with a ²³⁹Pu source are shown as: (1) the time resolutions became better gradually with the increase of the quantities of SiPMs connected in series; (2) when the number of 12 SiPMs was fixed, the time resolutions got worse as the number of parallel branches were increased; (3) the detector time resolutions can be effectively improved by using a fast-time plastic scintillator and adding its thickness; (4) to improve the positional uniformity of the time resolution, the scintillator with larger dimensions than beam spots should be used; (5) a resolution less than 131 ps was achieved with a 0.1 cm thickness EJ232 scintillator detector read out on each side by 12 SiPMs conncted in series. This study has importance significance for the upgrading of the start detector at RIBLL2.