The present invention provides a fluidized bed reactor and its use for producing olefins from oxygenates, the fluidized bed reactor comprises: a reaction zone located in the lower portion of the fluidized bed reactor and comprising a lower dense phase zone and an upper riser, wherein the dense phase zone and the riser are connected with each other transitionally; a separation zone located in the upper portion of the fluidized bed reactor and comprising a settling chamber, a fast gas-solid separation means, a cyclone and a gas collecting chamber, wherein the riser extends upwardly into the separation zone and is connected at its outlet with the inlet of the fast gas-solid separation means, the fast gas-solid separation means is connected at its outlet with the inlet of the cyclone via a fast gas passage, the cyclone is connected at its outlet with the gas collecting chamber, and the gas collecting chamber is located below the reactor outlet and connected therewith; and a catalyst recycle line for recycling the catalyst from the settling chamber back to the dense phase zone, a catalyst withdrawl line for withdrawing the deactivated catalyst from the settling chamber and/or the dense phase zone to the catalyst regeneration means, and a catalyst return line for returning the regenerated catalyst from the catalyst regeneration means to the dense phase zone.