A rank adaptive beamforming scheme for Long Term Evolution (LTE) system with imperfect channel state information (CSI) is proposed in this paper, where the rank of user equipments (UEs) is adapted based on two configurable thresholds: channel quality indicator (CQI) threshold and channel eigenvalue ratio threshold. In this scheme, the channel eigenvalue ratio threshold varies with channel estimation error in contrast to previous fixed threshold in rank adaptation. Moreover, the adaptation parameter of eigenvalue ratio threshold can be optimized based on hybrid automatic repeat request (HARQ) feedback information and current transmission mode. By introducing these configurable thresholds and parameter optimization, this scheme can achieve more accurate beam-forming rank adaptation and resist the performance loss due to imperfect CSI. Simulation results show that our proposed rank adaptive beamforming scheme can obtain tradeoff between system robustness and spectrum efficiency.