A method and apparatus are disclosed for updating a beam set of a UE (User Equipment) in a cell of a wireless communication system, wherein a base station applies beam-forming for transmission and/or reception in the cell and there are at least two beams in the cell. In one embodiment, the method includes the base station determines the beam set of the UE. The method also includes the base station monitors an uplink transmission from the UE via the beam set. The method further includes the base station sends a signaling to the UE for triggering at least an aperiodic reference signal transmission if no DM RS associated with the uplink transmission is detected via at least one beam in the beam set. In addition, the method includes the base station updates the beam set according to at least a beam or beams via which the aperiodic reference signal is received from the UE.