Multiuser Multiple input multiple output (MIMO) systems are now having more and more radio frequency (RF) chains, with larger capacity and at the same time higher energy consumption. With random data arrival, it is desired to turn off RF chains to save energy according to the traffic variations. In this paper a low-complexity traffic-aware scheme is proposed, whereby RF chains and users are selected at each frame based on the channel quality and the data queue-length. Particularly, the number of active RF chains is determined by comparing the current queue-length to the predefined thresholds, the values of which are able to control the tradeoff between the energy saving and quality of service, i.e., delay. Simulation results show that the proposed scheme saves more energy compared with conventional schemes which is designed regardless the traffic variations, and the saving gain increases when the average traffic load decreases.