It often needs to detect fluorescence signals in science research, such as atomic optics, quantum communications, biomedicine, food safety and laser measurements. However, the fluorescence signals are always very faint and difficult to measure. Therefore, a measurement system was developed to detect the extreme faint fluorescence signals. A Si-photomultiplier tube was used as a detection element in the measuring system. A trans-impedance amplifying circuit was designed to amplify the weak current signal. The circuitry was simulated by MultiSim software. Finally, the system was completed to detect the optical field. The experimental results show that the system can well meet the measurement requirements of the very faint light field distribution information.