The present invention provides a method for wireless backhaul frequency band flexible allocation. It is able to use the uplink band of FDD band as the wireless backhaul uplink and downlink band of small cells. When the status is having a plurality of FDD bands and a heterogeneous network with big cells and one or more small cells, the present invention is able to efficiently use uplink band of the FDD band as the wireless backhaul band of the one or more small cells.