Due to the compact nature, hearing aids are often equipped with only a small number of microphones. Such a restriction brings a conflict between noise reduction and spatial cue retention in binaural beamformers (BFs). To alleviate this conflict, we design a parametric binaural (PaBi) BF from the viewpoint of auditory perception. In human hearing, the binaural cues are frequency selective, i.e., the interaural phase difference (IPD) dominates at low frequencies while the interaural level difference (ILD) dominates at high frequencies. Accordingly, we construct a set of parametric IPD and ILD constraints to establish a novel cost function, which is then solved by the semi-definite relaxation strategy. By adjusting the involved parameters, a good trade-off between noise reduction and spatial cue preservation can be achieved. Moreover, the PaBi BF breaks through the degree-of-freedom limitation of existing methods. Experimental results show the superiority of the proposed method.