The previous experimental study illustrated that a direct laser scribing on polyimide (PI) induced the layered graphene, which opened a new door of producing graphene associated materials. However, underlain conversion mechanism is unknown, especially from theoretical perspectives. In this paper, we performed the molecular dynamics (MD) simulation of this mechanism under extreme conditions of the high pressure and temperature which mimic the ones generated by the laser induction. We investigated this process using the ReaxFF potential in a nanosecond time scale. We found out that at a pressure of ~ 3 GPa and temperature of > 2400 K generated from the NVT processes, the layered graphene clusters were crystallized from PI without metal catalysts, which is consistent with the previous experiment. However, if simulated using the NPT processes with a pressure of 27 MPa, the PI were decomposed into small molecules, and no graphene layers were observed. Furthermore, by analyzing the number of the carbon rings and the pair distribution function of them we quantified the crystallinity of the graphene clusters. Through tracing the emission of the small molecules such as H 2 O, N 2 , H 2 , CO we propose the possible reaction paths for both of the NVT and NPT processes.