Abstract: The precise and accurate knowledge of energy resolution function of photo-neutron source is mandatory for the experiment broadening of resonance cross section data and the determination of resonance parameters. The target system model of the TMSR photoneutron source (TMSR-PNS) was built in the framework of Geant4 Monte Carlo toolkit, and used to simulate the process of generation and transportation of neutrons in the target system in order to obtain the neutron TOF (time of flight) distribution. The RPI energy resolution function was used to fit and analyze the time distribution of the moderated neutrons of the TMSRPNS. A set of parameters were obtained for determining the energy resolution function of the TMSRPNS.