Abstract: In order to investigate the influence of the deflector layout on the performance of mine-used main fan diffuser, the RNG k-ε turbulence model was applied to numerical simulation of the internal flow fields under three inlet velocity conditions with and without deflector. The influence of the central angle and starting angle of the deflector on the performance of the diffuser was investigated. The results show that for the diffuser without deflector, a serious flow accumulation phenomenon exists inside the flow field, which restricts the performance improvement of the diffuser; when the center angle of the deflector is small, the larger the starting angle of its installation is, the higher the diffusion efficiency of the diffuser under the three inlet velocities considered, however the secondary flow due to the uneven pressure on the upper and lower sides of the deflector head appears at some starting angles, which results in an increase in flow losses. As the center angle of the deflector increases, the flow accumulation inside the diffuser improves gradually, and the secondary flow also gradually weakens. When the center angle reaches 60°, the diffusion efficiency is the highest.