Random circuit breaker (RCB) model is a powerful tool to investigate the formation and rupture processes of conductive filaments which occur in unipolar memristor devices. However, the existing RCB models do not integrate the time and thermal parameters, which downgrades significantly the model accuracy in emulating the dynamics of conductive filaments under pulse stimulus. Meanwhile, current research lacks detailed discussions about the above-mentioned problems. In this paper, a SPICEbased optimized RCB model is proposed to explore the unipolar resistive switching characteristics of memristor devices under pulse stimulus. Compared with the original RCB model, the set and reset transitions of each breaker in the proposed model are assumed to be dominated by the thermal heat, which introduces time variable by cascading the thermal equivalent circuits on the original main breaker network and thus allows to explore the filament formation and rupture dynamics along with time. It can simulate all unipolar memristor device operations realized by the original RCB model and is accurate enough to interpret effectively the novel features arising from pulse stimulus such as nonlinear current-voltage relation, multi-states storage capacity, etc.