Abstract: A \gamma ball array detector sub-trigger system is designed for the external target experiment of the Cooler Storage Ring of Heavy Ion Research Facility in Lanzhou(HIRFL-CSR). According to the structure of the \gamma ball array detector and the detected signal characteristics, the system quickly extracts the physical events of effective hits. Considering that there are many readout channels in the \gamma ball array detector, the sum of the physical events information of each channel is large. It requires a reliable, high-speed, long-distance data transmission structure. So the \gamma ball array detector trigger system adopts a master-slave structure, and it's designed based on optical communication technology. Moreover, the sub-trigger system is divided into the uplink and downlink sub-trigger systems. It aims to ease the complexity of the logic algorithm of the \gamma ball array detector sub-trigger system. Furthermore, the flexible reconfiguration of the ball sub-trigger system is realized based on the online logic reconfiguration of the overall Field Programmable Gate Array(FPGA) and the hierarchical organization of the trigger logic. The results show that the signal source test can generate the correct sub-trigger pulse signal and the bit error rate(BER) of the optical transmission link of the trigger judging system test below 2\times 10^-14 . The \gamma ball array detector sub-trigger system works well.