We report the design and construction of the trigger system for COMET Phase-I experiment of searching for cLFV of muon-electron conversion. A robust, intelligent and flexible trigger system of COMET Phase-I experiment is critical to cope with background hits rating a few MHz in Cylindrical Drift Chamber detector of COMET experiment. To establish robust links between central trigger and timing system with detector readout system, custom serial link rating 4.8Gbps and the receiver board for trigger and timing information are designed and produced. The primary trigger signal will be generated and processed to make intelligent decision using decision tree on conversion signal, through custom-made processing system. The performance of the central trigger system and its test results under beam condition will be reported also.