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Highlights • Both of DBD plasma deposition and direct fluorination can suppress the surface charge accumulation and increase the vacuum flashover withstanding voltage. • Due to the introduction of shallow traps and polar hydrophilic groups in the reaction process of the DBD plasma deposition and oxyfluorination, the surface conductivity and the decay rate of surface charges are increased. • The degradation of the surface modification effect is attributed to the unstable polar hydrophilic groups. Abstract Polystyrene (PS) as an industrialized insulating material is widely used in the pulsed power system, due to the good mechanical and electrical properties. However, the surface flashover of dielectric material limits the development of pulse power technology seriously. It is believed that the surface charge dynamics are related to the surface flashover closely. Therefore, in this paper, several surface modifications including plasma deposition assisted by dielectric barrier discharge (DBD) and direct fluorination (with/without O 2) are utilized to enhance the surface electrical properties of PS. The surface charge dynamics characteristics before and after surface modifications are investigated. The experimental results show the decays of surface charges for DBD plasma deposition and oxyfluorination become faster significantly. The surface potential of PS after DBD plasma deposition and oxyfluorination decreases to around 0 V over 100 s. The FTIR spectra indicates that a large amount of polar hydrophilic groups are introduced on the surface of PS samples by DBD plasma deposition and oxyfluorination, which are conducive to the dissipation of surface charges. The decay rate of surface charge after fluorination treatment without O 2 is also increased, which is associated with C F and COOH groups in the fluorination layer. Due to the rapid reduction of accumulated charges, the flashover voltage of all modified samples are increased by more than 30%. Furthermore, the stability of surface modification layer is compared. It is found that the stability of fluorination is better than oxyfluorination and DBD plasma deposition. [ABSTRACT FROM AUTHOR]