At present, artificial field sampling and laboratory instrument analysis are the main ways to test water toxicity, which has defects that sampling error is big, detection process is complex, fails to reflect the status of polluted water changes in time. It is difficult to meet the demand of the water environment monitoring development. Therefore, this paper designs a kind of water synthetic toxicity online detector based on luminescent bacteria. It has designed a photoelectric detection circuit in the hardware. The device with STM32F103ZGT6 as the core, using photomultiplier tubes to collect fluorescence signals and turning into a weak current signal, after a followup circuit amplification processing and A/D conversion transmit to MCU. Then communication with PC through RS232 and RS485. With related software system, it can be realtime monitoring and online intelligent analysis. The experiments show that it can be finished within 10 minutes, and it can effectively distinguish 1 mg/L of toxic substances. The detector has good sensitivity, accuracy and repeatability, which is suitable for most kinds of water synthetic toxicity online monitoring.