To improve the performance of the Stairmand cyclone, the effects of an expansion chamber on the flow field, the pressure drop and the separation efficiency were investigated numerically and experimentally. The experimental results showed that compared with the Stairmand cyclone, the cyclone with an upper expansion chamber worked better at low inlet velocity(less than 14 m/s in this study), while the cyclone with a lower expansion chamber achieved higher efficiency at a relatively high inlet velocity(14-20 m/s). The presence of an expansion chamber can generally result in a slight decrease in the cyclone pressure drop. The simulated results, which were used to further analyze the reason behind the experimental phenomena, suggested that the expansion chamber had insignificant effects on the tangential velocity profiles in the cylindrical part of cyclones. While in the cone part, the expansion chamber and the dipleg, the tangential velocity slightly decreased. Nevertheless, the expansion chamber decreased the possibility of the vortex end to sweep the wall and then reduce the particle re-entrainment. Therefore, installing the expansion chamber at a proper position could improve the separation performance of Stairmand cyclones. Both the experimental and simulated results represent a potential improvement of the Stairmand cyclone performance. To improve the performance of the Stairmand cyclone, the effects of an expansion chamber on the flow field, the pressure drop and the separation efficiency were investigated numerically and experimentally. The experimental results showed that compared with the Stairmand cyclone, the cyclone with an upper expansion chamber worked better at low inlet velocity (less than 14 m / s in this study), while the cyclone with a lower expansion chamber achieved higher efficiency at a relatively high inlet velocity (14-20 m / s). The presence of an expansion chamber can generally result in a slight decrease in the cyclone pressure drop. The simulated results, which were used to further analyze the reason behind the experimental phenomena, suggested that the expansion chamber had insignificant effects on the tangential velocity profiles in the cylindrical part of cyclones While in the cone part, the expansion chamber and the dipleg, the tangential velocity slightly decreased. Nevertheless, the expansion chamber decreased the possibility of the vortex end to sweep the wall and then reduce the particle re-entrainment. Therefore, installing the expansion chamber at a proper position could improve the separation performance of Stairmand cyclones. Both the experimental and simulated results represent a potential improvement of the Stairmand cyclone performance.