An experiment about asynchronous sampling was reported to monitor the 2.5 Gb/ s optical signal performance. Eye diagram, Q factor and BER were acquired by analyzing and calculating the sampling result. Compared with the eye diagram observed by oscilloscope, the experimental results matched perfectly. This report shows that the asynchronous sampling,with data analyzing and calculating, can monitor the optical signal performance well,it does not need lock recovery,and it is independent of data ratio and pattern.