A rectifier mounted at a bottom of a reactor includes a central layer (1), at least one interlayer (2) and an external layer (3), which are made of bird nest honeycomb ceramic, wherein porosities of the central layer, the interlayer and the external layer increase in turn. The central layer, the interlayer and the external layer are respectively provided in a central-layer cone shaped body, an interlayer cone shaped body and an external-layer cone shaped body which are installed concentrically, so as to form meshed honeycomb pores with different porosities at the bottom of the rectifier. The rectifier is made of bird nest ceramic with high density honeycomb pores, and a minimum density thereof is over 200 meshes.