The continuous-wave (CW) light obtained by the 1560 nm external-cavity diode laser (ECDL) is amplified to about 5 W by a erbium-doped fiber amplifier (EDFA). The periodically poled lithium niobate crystal and the periodically poled potassium titanium orde phosphate crystal are used to perform single-pass qusi-phase-mached frequency. The 780 nm laser power of about 336 mW and 210 mW, the frequency doubling efficiency about 7% and 4.4% are abtained. It shows that the good characteristic of single frequency output by monitoring the longitudinal mode of 780 nm laser. Additionally, the absorption spectra for Rb atom line D2 are scanned, and the tunable range of the 780 nm laser is more than 10 GHz. The ECDL is locked to 87Rb 5S1/2(Fg=2)-5P3/2(Fe=3) hyperfine transition via modulation-free polarization spectroscopy. Compared with frequency fluctuation of about 4 MHz for ECDL free running within 450 s, the residual frequency fluctuation can be squeezed below about 1.5 MHz after being locked.