Aiming at the requirements of internal field strength calculation of electrically large cabin under high intensity radiated field (HIRF), the sub-domain decomposition methods are used for accelerating the calculation of electromagnetic current iteration method. Moreover, multiple reflection field distribution with limited number of reflections are estimated and modified according to the energy conservation principle, as well as with the prior knowledge of reflection field random distribution. Therefore, fast computation of field intensity distribution of electrically large cabin can be realized. The results derived by the proposed method agree reasonably with the results obtained by using the numerical method. Finally, this fast iterative computation method is applied for electric field intensity analysis of oversized cabin with internal stirrer.