The peaking capacitor used for pulse compression in electromagnetic pulse (EMP) simulator often uses the laminate structure of electrodes and thin film dielectrics to achieve insulation at high voltage, and the surface flashover is the main insulation failure mode of the peaking capacitor. In this paper, the effect of key structural parameters of the peaking capacitor and the gas pressure on its surface flashover performance were experimentally studied under the condition that the insulation gas was SF₆ and the risetime of the applied voltage was approximately 30 ns. The experimental results show that: (1) the electrode thickness, the interspace around the electrodes and the surface coating ceuld not significantly change the surface flashover voltage of the laminated structure; (2) increasing the gas pressure could improve the flashover performance at low gas pressure; (3) the flashover voltage is nearly proportional to the number of polymer film layers; (4) the flashover voltage could be increased remarkably by increasing the extension length of the polymer film dielectrics. Based on the streamer theory, the above results are analyzed and discussed. It is considered that in inhomogeneous field the flashover initiates in the strengthened field region but the formation and development of the flashover channel is mainly determined by the background field on the flashover path. Therefore, reducing the electrical field at the triple point has little effect on the flashover performance but reducing the field on the flashover path can improve the flashover voltage of the laminated structure significantly.