In this study, a numerical study has been carried out to analyze the factors of the efficiency decrease at backward flow situation in an axial fan with adjustable blades. The analysis is done with the pitch angle of $36^{circ}$ on the forward flow and of $-26^{circ}$ on the backward flow. The numerical results show that the air flow rates of the pitch angle of $36^{circ}$ and $-26^{circ}$ are calculated to be cubic meter per min (CMM) and 212 CMM, respectively. The results are similar to the experimental results by Chang et al. The results had the maximum error of 10.6% compared with the experimental results. The fan efficiency decrease is caused by the fact that the axial fan used for this study was designed for the forward flow. As the results, the pitch angle of $-26^{circ}$ has induced the recirculation around the impeller blade, impeller cover and downstream. This recirculation caused the large decrease in total pressure coefficient. This turned out to be the main cause of the efficiency decrease.