A multi-core fiber laser with single-mode output is reported, which is mode selected by the proposed mechanism based on single-mode fiber. Using single-mode fiber combined with a total reflection mirror as mode selection element, the in-phase supermode always has much higher coupling coefficient than that of the other higher-order supermodes. As a result, high brightness single in-phase supermode output is obtained while higher-order supermodes are suppressed. The coupling coefficient can be optimized by choosing proper single-mode fiber core radius and adjusting the gap distance between multi-core fiber and single-mode fiber. The relationship between coupling coefficient and single-mode output power is investigated. The simulation results got from rate equation show that the output power increases with coupling coefficient, and the maximum optical-optical conversion efficiency is calculated to be 63.7%.