This work analyses the performance of multi-user massive MIMO relay networks, where the relay station is equipped with a large number of antennas. The combination of maximal-ratio-combining (MRC) and maximal-ratio-transmission (MRT) is utilized at the relay for low-complexity processing. Different from existing work, we consider the practical scenario with a large but finite number of relay antennas, limited number of users, fixed source power, and fixed relay power. Our work shows that the interference is neither negligible nor asymptotically deterministic, and it dominates the statistical properties of the signal-to-interference-plus-noise ratio (SINR). Via deriving the probability density function (PDF) of the interference power, analytical results on the sum-rate and outage probability performance of the multi-user relay network are obtained.