Numerical simulations of CAEP FIR FEL are made with a INEX (Integrated Numerical Experiment) model in which the transverse distribution is considered carefully. Compared with the results of simulations with Moute Carlo method, large differences between them are found. The performance of free electron lasers depends upon the characteristics of the electron beam. It is necessary to use this kind of INEX simulations in order to accurately calculated and predict the lasing performance of free electron lasers.