Abstract: In order to obtain the influence of laser ablation on the electrical properties of quartz fiber/cyanate composite and reveal the influencing mechanism, it is great significance for the evaluation and analysis of wave transmission performance and thermal protection design under extreme harsh thermal environment. Using laser as an external heat flow loading method, the laser ablation experiment of quartz/cyanate composite was carried out in this paper, and the dielectric constant before and after the experiment was tested. In order to analyze the change mechanism of dielectric constant, the quartz fiber/cyanate composite surface products after laser ablation and quartz fiber/cyanate composite were tested by transmission infrared spectroscopy and XRD. The ablation surface of quartz fiber/cyanate composite irradiated by laser was observed by SEM. The thermal weight loss of cyanate and quartz fibers were also tested. The results show that the dielectric constant is about 6 and nearly 1 time higher than the initial state in the range of 7-18 GHz. The influence mechanism of laser ablation on the electrical properties of quartz fiber/cyanate composite is as follows:The absorption of laser energy makes the cyanate thermal decomposition and cracking, the formation of carbon black, which is conductive ability and the chain state in situ on surface, the dielectric constant of the ablation sample increases, the radar wave absorption capacity of quartz fiber/cyanate composite that is ablation state will be enhanced under this condition. At the same time, the rough surface and the loose state formed by ablation will enhance the reflection and scattering of electromagnetic wave, which can further weaken the radar wave transmission capability.