The invention discloses an optical network synchronization testing method and system, and solves a problem that the synchronization references of different places are not consistent. The system comprises a clock interface unit, an optical monitoring channel processing unit, a clock reference source, and a synchronization testing instrument. The method comprises the steps: generating a frequency reference signal by using a clock reference source, generating a time reference signal as an external synchronization signal by using the synchronization testing instrument, inputting the external synchronization signal into a clock interface unit of a local network element, generating a line synchronization signal, then packaging the signal to an optical monitoring channel signal, and transmittingthe signal to a line network element; setting the line network element, and transmitting the line synchronization signal through a line optical monitoring channel processing unit until the line synchronization signal is transmitted to a local optical monitoring channel processing unit; setting the local optical monitoring channel processing unit to recover the line synchronization signal from theoptical monitoring channel signal, then recovering the external synchronization signal as a tested signal by the clock interface unit, and inputting the tested signal into the synchronization testinginstrument. The method improves the accuracy, avoids the constraint of onsite conditions, and reduces the cost.