This paper introduces the maloperation of C1 unbalance protection caused by disturbance in Xing-an HVDC project in recent years and analyzes the disturbance waveform. The internal structure of DC filter C1 capacitor, the principle and configuration of C1 unbalance protection are analyzed. The function of integration module INT that is vital to C1 unbalance protection program is analyzed. To avoid the C1 unbalance protection maloperation again, according to the development process of C1 capacitor unbalance faults, a modification is proposed for C1 unbalance protection. In the modification, the dynamic segment uses counting scheme to limit the impact of disturbance on the protection and to avoid maloperation. According to the overvoltage level caused by the C1 capacitor unbalance faults, different action strategies are arranged in the acceptable range of overvoltage level. When the number of short-circuit small strings is small and the overvoltage level is relatively low, alarm strategy is adopted; when the number of short-circuit small strings is big and the overvoltage level is relatively high, trip strategy is used.