Abstract: This work mainly focuses on the evolution mechanism of Frank loops in austenitic stainless steels under neutron irradiation by computer simulation. Molecular dynamics and mean field rate theory were employed. Firstly, a rate theory model was constructed to model the evolution of the neutron irradiation induced Frank loop in austenitic steels; Secondly, this model was parameterized by molecular dynamics simulation, and the model was validated by comparing the simulation results of dislocation loops with the experimental data in steels under electron and neutron irradiation conditions. Based on our results, the effect of survival defects under cascade collision was studied, and the survival fraction, the cluster fraction of defects and the fraction of 4-interstitials-clusters are the mainly factors for the evolution of Frank loops. The fraction of interstitial clusters which size are smaller than 4 has no effect on the Frank loop evolution under neutron irradiation.