When an HVDC power transmission system is operating in isolated island mode, if a system failure occurs, the frequency of the AC system will change over a large range. This will increase the current of the AC filter element. In this paper, the influence of the frequency change on the rated current of the AC filter components is studied by analyzing the frequency change under the conditions of the island fault of the HVDC transmission project. Based on the characteristics of the frequency-supplied filter of a single-tuned filter, a method for calculating the settling value under multi-tuned filter frequency fluctuation is proposed. On this basis, the influence of the conditions of the AC system and the parameters of the filter on the fluctuation of the device setting with frequency fluctuation is analyzed. A method for evaluating the capabilities of AC filter equipment under island conditions is proposed. Simulation results verify the effectiveness of the proposed method. This work is supported by Science and Technology Project of China Southern Power Grid Co., Ltd. (No. ZBKJXM20170065).