Abstract: The time structure of the synchrotron light is same as beam signal in the storage ring. Therefore, the bunch length measurement can be done by measuring the optical pulse in frequency-domain. Based on the principle, the bunch length measurement system was developed using harmonic method. The system used the high-speed photoreceiver that converted optical signal to electrical signal, collected four of the signal frequency components, and then calculated the bunch length. The hardware and software design of the system was presented. The variation of bunch length with beam current was measured using the system, which was consistent with the measurement result of the streak camera.