The invention provides a method for calculating a power fluctuation peak value of an alternating current tie line under impacts of direct current phase changing failure. The method includes the steps of firstly, obtaining a direct current power curve within a direct current phase changing failure period, and calculating n-circuit direct current equivalent phase changing failure lasting time tau and natural oscillation frequency omega and the damping ratio zeta of a large regional grid regional oscillation mode; secondly, building a power oscillation linearization second-order model of the alternating current tie line; thirdly, obtaining the power fluctuation peak value, the peak value power transfer ratio and the peak value time of the alternating current tie line. Compared with the prior art, the method for calculating the power fluctuation peak value of the alternating current tie line under the impacts of the direct current phase changing failure has the advantages that various parameters of the power fluctuation peak value of the alternating current tie line can be rapidly and accurately obtained, a technical person can easily determine the tie line operation power limit and a power grid operation mode, and take effective power control and protection measures, and the safe and stable operation level of a power system can be improved.