Distributed antenna system (DAS) can improve the spectral efficiency of the cellular system, especially at the cell edge. In the paper, we consider the cognitive radio technique to further improve the cell edge performance of the DAS. We consider the following scenario: multiple secondary users (SUs) share the same spectrum with the primary user (PU) at the cell edge, and one or two SUs are selected to forward signals for the PU. First, we study the transmit scheme using MRT, and the downlink capacity of the PU is analyzed. Then, we generalize the idea of dynamic relay selection by allowing one or two SUs to cooperate. The dynamic selection scheme can be realized based on the optimal signal noise ratio (SNR). Simulation results show that dynamic selection of one relay can achieve better performance than the static selection of one, and dynamically selecting one SU outperforms the dynamic selection of two because of the increasing of the resource consumption along with the number of relay nodes. With different choices of the channel state information (CSI), the large-scale fading achieves a better tradeoff between the performance gain and processing complexity. The effect of different selection threshold on the average capacity is also given.