3-demension flow field in inlet box of G47-3 No.8D fan were studied numerically by using CFD, which showed that there was a small and a pressure loss region formatted by strong eddy current and reflux. According to the distribution of flow field, reformation and simulation were carried out on the inlet box. The results show that the big eddy current in old model has changed into two eddy current which are small in strength and size, the small eddy current in the old model has disappeared, as well as the reflux has been suppressed to some extent.