In this paper, we study a relay selection (RS) problem in multi-user two-way cooperative relaying systems. We consider a more practical scenario in which multiple users, multiple relays and a single destination are involved in the two-way network. In this paper, the code division multiple access (CDMA) system with non-orthogonal spreading sequences is employed to handle the multiuser interference. Relay selection based on maximizing the SINR of the worse link is proposed in this research. Besides, aiming at mitigating the interference, we consider the design of linear filter at each relay such that the minimum SINR of the worst link in the two-way transmission is maximized. The result shows that the linear filter is similar to minimum mean-square error (MMSE) detector. Furthermore, we simulate the proposed scheme with several different parameters such as the numbers of users and relays, and the length of spreading sequences. Also, we compare the proposed RS method with random RS approach, and the result shows that our proposed method has better performance in terms of the bit error rate (BER).