Embodiments of the present disclosure disclose a Physical Uplink Control Channel (PUCCH) resource allocation method, a network side device and a communications system, wherein the method includes: when a new user equipment (UE) access a cell in a case that idle PUCCH resources of the cell are insufficient or all PUCCH resources of the cell are fully occupied, determining a preempted UE among UEs already accessing the cell; allocating PUCCH resources of the preempted UE to the new UE, reconfiguring the preempted UE and notifying the preempted UE to release the PUCCH resources.