As an indispensable method of constructing ubiquitous communication network, satellite communication technology has received significant attention in both industrial and academic areas recently. In this paper we investigate the performance of an integrated satellite-terrestrial cooperative network in the presence of co-channel interference, which consists of a satellite source having a single antenna, a terrestrial relay equipped with multiple antennas to assist satellite signal transmission, and a single-antenna user corrupted by multiple co-channel interference. On the assumption that the user receives the signals from direct link and relaying link with decode-and-forward protocol, the output signal-to-interference-plus-noise ratio of the user with the maximal ratio combining scheme is firstly obtained. Then, according to the Meijer-G function, we derive the moment generating function of the destination and the relay, where the satellite links are assumed to experience Shadowing-Rician fading while the terrestrial links undergo Rayleigh fading, and the analytical average symbol error rate expression for the considered system is obtained. Finally, the simulation results demonstrate the effectiveness of the theoretical analysis and reveal theinfluences of antenna number, interference number, and modulation schemes on the system performance. Therefore, our work provides useful guidelines for the engineers in designing the integrated satellite-terrestrial cooperative networks for future satellite mobile communication.