The sampling function， concentrated in EOVT/EOCT and controlled by merging unit， usually uses the fixed sample frequency and the traditional AC measurement method that bring an over-error. An adaptive filtering AC measurement method， which uses a set of adaptive filters to measure and analyze the measured signal， is designed by the gradient descent approach. Simulation results demonstrate that the convergence period of the adaptive filter is less than 6 fundamental cycles in the steady AC signal measurement. This method has the advantages of high accuracy and less calculating time， and can satisfy the precision and real-time demand of telecontrol and metering. A solution to analyze high-order and non-integer order of harmonics is also given. This method proposed has already been used in a designed IED.