Huang, X.Y. Jiao, Y. Wei, Y. JACoW Publishing Geneva, Switzerland In this paper we present a step-by-step numerical simulation of fast orbit feedback (FOFB) system in synchrotron light source. Dynamic models in each stage are modelled based on measurement results or reasonable assumptions. With the simulation, we can estimate the performance of the FOFB system, as well as the effect of the element errors and orbit response matrix errors. The influence of the singulars used in the inverse response matrix can also be investigated.