A laser-diode-pumped two-frequency solid-state laser with tunable frequency difference is investigated, the principle of coupled-cavity is presented. Single-longitudinal mode oscillation is obtained from a coupled-cavity setup. Two quarter-wave plates are inserted to lift the degeneration of frequencies in orthogonal polarizations. 34 mW output power of two-frequency laser with tunable frequency difference is achieved. The beat note frequency can be continuously tuned from 0 to 1.1 GHz.