In this paper, we study the communication between terrestrial source and destination terminals via a geosynchronous (GEO) multi-beam satellite in satellite mobile communication (SMC) system. The end-to-end transmission is modeled by a relaying process, in which inter-beam interference plays a key role due to frequency reuse among adjacent beam cells and side-lobe leaking of practical beam antenna. Through theoretic analysis we obtain the closed-form expressions of end-to-end outage probability and system achievable throughput as the major performance metrics. Both theoretic and simulation results, which coincide well with each other under diverse beam frequency planning, suggest that reuse factor of 1/3 is an appropriate compromise between system throughput and reliability in the GEO SMC system.