This work provides a comprehensive scaling law and performance analysis for multi-user massive MIMO relay networks, where the relay is equipped with massive antennas and uses MRC/MRT for low-complexity processing. CSI error is considered. First, a sum-rate lower bound is derived which manifests the effect of system parameters including the numbers of relay antennas and users, the CSI quality, and the transmit powers of the sources and the relay. Via a general scaling model on the system parameters with respect to the relay antenna number, the asymptotic scaling law of the SINR as a function of the parameter scalings is obtained, which shows quantitatively the tradeoff between the network parameters and their effect on the network performance. In addition, a sufficient condition on the parameter scalings for the SINR to be asymptotically deterministic is given, which covers existing studies on such analysis as special cases. Then, the scenario where the SINR increases linearly with the relay antenna number is studied. The sufficient and necessary condition on the parameter scaling for this scenario is proved. It is shown that in this case, the interference power is not asymptotically deterministic, then its distribution is derived, based on which the outage probability and average bit error rate of the relay network are analysed.