Abstract: The software synchronization algorithm in asynchronous optical-sampling optical performance monitoring is researched for the purpose of eye diagram reconstruction and <em<Q</em< value monitoring. Using chirp-<em<z</em< transform (CZT) to zoom local discrete Fourier transform spectrum, the spectral resolution can be improved and the sampling step size can be derived accurately. Simulation experiments show that the CZT based software synchronization algorithm is a precise way to reconstruct eye diagram and to calculate <em<Q</em< value. The algorithm has higher accuracy and robustness than current algorithms. The required operation time is also reduced by 40%. This scheme is featured by high speed and accuracy, and has promising application in optical-sampling optical performance monitoring systems.