Deionized water, commonly used as insulating dielectric in pulse-forming lines or transmission lines of high-power pulse devices, offers advantages such as a high dielectric constant, high breakdown strength, good self-healing properties, and low cost. However, the solid insulation barriers in deionized water, which support the inner cylinder and provide physical isolation between different dielectrics at the front and rear ends, are often the weak points in high-voltage insulation systems. To investigate the surface flashover characteristics of typical solid insulation materials in deionized water, this study utilized a high-voltage insulation experimental platform with a maximum operating voltage of approximately 900 kV and a pulse rise time of about 100 ns. The study focused on four common solid insulation materials: MC Nylon, polymethyl methacrylate (PMMA), cross-linked polystyrene (CLPS), and high-density polyethylene (HDPE). Using circular plate electrodes and cylindrical samples, the experiments examined the effects of sample material, thickness, voltage duration and surface roughness on flashover voltage and electric field strength. Results show that as the sample thickness increases from 0.5 cm to 2 cm, the flashover voltage increases linearly, while the flashover field strength decreases exponentially. For different materials, the flashover voltage and field strength follow the order: MC Nylon ≥ PMMA > CLPS > HDPE. As the voltage application time shortens, the flashover voltage gradually increases. When the voltage application time is within 100 ns, the flashover voltage remains basically stable. Furthermore, when the surface roughness of the solid material increases from 1.6 μm to 12.5 μm, no significant change in flashover field strength is observed. Based on a comprehensive analysis of flashover field strength data and impact resistance characteristics, MC Nylon demonstrates the best overall.