A wavelength-tunable laser output at 589 nm with high conversion efficiency based on sum-frequency generation by using the technique of single-wavelength extra-cavity resonance is achieved. The two fundamental wavelengths are 1583 nm and 938 nm and the nonlinear crystal is the period-poled lithium niobate. After the frequency of 1583 nm laser was locked to the cavity mode and the frequency of 938 nm laser was scanned, a 589 nm laser output with power of 4.96 mW and wavelength tuning range of 7 GHz was obtained and the stability of the output power is improved effectively with the help of servo feedback loop technique of acousto-optic-modulator. Finally, based on this laser, the saturated fluorescence spectrum of sodium D2 line in the temperature range of 348—413 K (75—140 ℃) were measured. The Doppler-free structures of D2a, D2b and crossover lines on Doppler background were observed, which can provide reference signals for the frequency locking of 589nm laser.