The effects of digital filter group delay on efficiency of gust load alleviation are studied in the gust load alleviation control system of a two-dimensional airfoil. Firstly, aeroelastic model is established and the delay system is transformed into an equivalent delay-free system. Secondly, Guest generator，digital filter and time delay Linear quadratic(LQ) controller are designed. Finally, the system is studied through numerical simulation and wind tunnel experiments. Results demonstrate that a small amount of group delay of the filter may improve the performance of the control system and increase efficiency of gust load alleviation, but the increase of the filter group delay reduces the gust load reduction efficiency significantly and even leads to the failure of controlling.