The invention relates to a reaction unit for preparing low-carbon olefins, mainly solving the problem of low yield of low-carbon olefins in the prior art. The reaction unit mainly comprises a rapid fluidized bed reaction zone 2, a rough-cut cyclone 4, a disengager 21, a gas-solid distribution zone 7, a downer reaction zone 9, a regenerator 11, and a riser reaction zone 20, wherein the outlet of the riser reaction zone 20 is connected with the disengager 21, the bottom of the disengager is connected with the regenerator 11 through a regeneration standpipe 26, the regenerator 11 is provided with two catalyst outlets, one catalyst outlet is connected with the riser reaction zone 20, the other catalyst outlet is connected with the downer reaction zone 9, the outlet of the downer reaction zone 9 is connected with the rapid fluidized bed reaction zone 2, the outlet of the rapid fluidized bed reaction zone 2 is connected with the rough-cut cyclone 4, a gas-phase outlet pipeline 5 of the rough-cut cyclone 4 is connected with the disengager 21, the dipleg of the rough-cut cyclone 4 is connected with the gas-solid distribution zone 7, and the gas-solid distribution zone 7 is connected with the downer reaction zone 9. The reaction unit disclosed herein well solves the problems and can be used in the industrial production of low-carbon olefins.