Abstract: In order to deeply study the feasibility and transient-state safety characteristic under propulsion mode of bimodal space reactor with heat pipe (HP-BSNR) proposed in this paper, the mathematics model was established based on the preliminary reactor system parameters obtained from steady-state code and the simplified structure of reactor core. Then a transient-state safety analysis code TTHA_HPBSNR for HP-BSNR was also developed. With the developed transient-state safety analysis code, typical transient accident conditions in HP-BSNR including reactivity insertion and loss of propellant were analyzed. And the influence of reactor core key parameters on the transient-state safety characteristic was also studied. The results show that the core power can reach a new steady state under the condition of reactivity insertion accident and the maximum fuel temperature does not exceed the safety limitation because of the negative feedback mechanism. Besides, under the condition of losing propellant, HP-BSNR can shut down automatically depending on the negative feedback mechanism and the value of negative feedback coefficient determines the response time of automatic shutdown. In conclusion, compared with the reactivity insertion accident, the accident of losing propellant has a larger threat to HP-BSNR safety operation.