The electromagnetic transient process of flexible direct current (DC) grid is complex. It easily causes overvoltage when a single-phase-to-ground fault occurs in converter station,which seriously affects the safe and stable operation of the system. The converter station is divided into alternating current (AC) fault area and DC fault area by single-phase-to-ground fault. Firstly,the compound sequence network is used to quantitatively analyze the mechanism of AC overvoltage and characteristics of fault zero-sequence components. Then,the propagation law of fault sequence components from AC side to DC side is deduced based on the control equation of converter,which reveals that the fault zero-sequence component is the main factor leading to overvoltage at the DC side and other converters. In addition,a DC overvoltage control strategy is proposed to suppress DC side overvoltage,and the selection method of controller parameters is given. At last,a test system is built on the basis of PSCAD simulation. The simulation results show the correctness of fault mechanism deduction and the effectiveness of overvoltage suppression strategy which improves the safety and stability of power system after single-phase-to-ground fault in the converter station.