It is of great significance for the development and application of pulse power technology to explore ways to improve the vacuum breakdown threshold of metal surface. Based on the theoretical analysis of electron on metal surface, the variation of surface electric field of cathode rod with diode voltage is calculated by finite element method. Experimental system is designed and experimental research is carried out. The experiment compares the vacuum breakdown threshold of TC4 of titanium alloy under different kinds of dielectric films (film thickness 30?60 μm), when the pulse width is about 30 ns and the gap between cathode rod and anode tube is 12 mm. The TC4 cathode rods with a surface roughness R_z (maximum outline height) of 0.8 μm are plated with epoxy resin and acrylic acid film respectively. The experimental results show that the breakdown threshold of the acrylic acid coated cathode rod is about 505 kV/cm. Compared with the uncoated cathode rod, the breakdown field strength is increased by about 20.6%; The TC4 cathode rods with surface roughness R_z of 0.2 μm are plated with polyimide film and polyether-ether-ketol film respectively. The experimental results show that the breakdown threshold of the polyimide film coated rod is about 584 kV/cm. The breakdown field strength is enhanced by approximately 28.1%, relative to the non-coated cathode rod. Therefore, coating acrylic acid film and polyimide film on the surface of metal cathode rod can effectively improve the vacuum breakdown threshold of metal rod.