An online assessment method for AGC regulation based on WAMS is proposed. Firstly, this paper puts forward the assessment index system which integrates the indices in time scale and state dimension, thus makes a comprehensive appraisal for AGC units. Then responding to the high sampling rate and high precision characteristics of WAMS data, the control process of AGC regulation is divided into three segments:fast segment, assessment segment and time-out segment. Different methods are applied in different segments, so the calculation amounts and data storage can be efficiently declined to meet the requirements of fast online evaluation. The evaluation system has been applied in Hunan Power Grid and the results verify the feasibility and efficiency of proposed method.