An adaptive window function for distorted power measurement is introduced in this paper. It can automatically provide the optimal window function according to the measured signal. Thus, the applied window function is always optimal in the sense of the zeros of its frequency response corresponding to the undesired measured signal spectrum components which will result in the harmful trunction errors. As a result, the harmful truncation errors of asynchronous sampling and the influence of interharmonics in the measured signal spectrum can be totally eliminated. The correctness, accuracy, and applicability of the proposed method have been verified both theoretically and by extensive simulations.