The inlet velocity affects the flow field of a cyclone separator，and the shape of the inlet pipe is one of the most important factors that may change the velocity. With the optimization for the shape or structure of inlet pipe，cyclones with better performance can be designed. Here the separation efficiency and the flow field of two double-inlet separators with straight inlet pipes and a vortex-tube inlet are analyzed and compared. It is shown that the total separation efficiency of the cyclone with vortex-tube inlet is 1.5% higher than that with the straight inlet，and the pressure drop of the former is 25% lower. The reason for lower pressure drop is that in the vortex-tube inlet cyclone the flows in the inlet pipe and in separation space are both vortex patterns，alleviating the energy consumption when the two streams collide. The higher separation efficiency is resulted from that the tangential velocity in the separator with vortex-tube inlet is 15% higher than that with the straight-pipe inlet. Thus the cyclone with vortex-tube inlet can reduce the cut size and improve the separation efficiency. According to the vortex theory，the local vortex in the inlet pipe of the vortex-tube inlet separator will create an induced velocity field，which increases the tangential velocity in the separation space without additional energy loss. Thus，using vortex-tube inlet and increasing the vortex intensity should be effective to improve the separation performance.