The influence of structural parameters on laser wavelength continuous tuning range in external cavity semiconductor laser with Littrow configuration is studied. The influence of allowed machining errors at the key points such as the mirror rotation axis on laser wavelength continuous tuning range is calculated. The details of designing external cavity semiconductor laser are presented. An external cavity semiconductor laser is designed, and with a commercial semiconductor laser tube it can produce qualified 780 nm laser, with line width less than 1 MHz and wavelength continuous tuning range of 3 GHz. The semiconductor laser is frequency stabilized by use of the third and fifth harmonics of Rb saturated absorption spectra. The method for optimizing the laser frequency short-term stability is proposed, and the choice of modulating depth is explained theoretically in detail. With the proposed frequency stabilizing method, the stability of laser reaches 10-12 at 780 nm wavelength.