The invention discloses a power wide-area time lag controller design method based on a Jensen inequation. The method is used for improving time lag stability margin of a closed-loop system in a wide-area system design time lag stability controller. The method comprises the steps that 1, to-be-solved system first-generation controller parameters are randomly generated; 2, a state space model of theclosed-loop system without time lag is established; 3, for the stable closed-loop system without time lag, dichotomy is utilized to iteratively call a Jensen inequation time lag criterion to solve allowed time lag stability upper limits; 4, intersection, variation and selection operation of a differential evolution algorithm are utilized to generate new-generation controller parameters; 5, whether the number of iterations reaches an upper limit or not, or whether the difference between the previous optimal allowed time lag stability upper limit and the later optimal allowed time lag stabilityupper limit is smaller than a designated value or not is judged, if yes, the step 6 is entered, or otherwise the step 2 is returned to; and 7, the optimal controller parameter and the time lag stability upper limit thereof are output, and operation is ended.