This paper proposes a downlink multi-user transmission scheme for the amplify-and-forward (AF)-based multi-relay cellular network, in which Tomlinson-Harashima precoding (TH precoding) and interference alignment (IA) are jointly applied. The whole process of transmission is divided into two phases: TH precoding is first performed at base-station (BS) to support the multiplexing of data streams transmitted to both mobile-stations (MS) and relay-stations (RS), and then IA is performed at both BS and RSs to achieve the interference-free communication. During the whole process, neither data exchange nor strict synchronization is required among BS and RSs thus reducing the cooperative complexity as well as improving the system performance. Theoretical analysis is provided with respect to the channel capacity of different types of users, resulting the upper-bounds of channel capacity. Our analysis and simulation results show that the joint applications of TH precoding and IA outperforms other schemes in the presented multi-relay cellular network.