How has the universe been formed and evolved? The Milky Way, our own galaxy, consists of several tens of billions of stars. How has it been formed and evolved? In the whole history of human civilization, these basic and profound questions inspirit people to explore the nature. The optical spectrum contains abundant physical information of distant celestial objects, and acquiring spectra of a large number of celestial objects is desperately needed in astronomy, which touches various cutting-edge researches of contemporary astronomy and astrophysics. The scientific goal of LAMOST focuses on the extragalactic observation, structure and evolution of the Galaxy, and multi-wave identification. The spectroscopic survey carried out by LAMOST of tens of millions of galaxies and others will make substantial contribution to the study of extra-galactic astrophysics and cosmology, such as galaxies, quasars and the large-scale structure of the universe. Its spectroscopic survey of large number of stars will make substantial contribution to the study of stellar astrophysics and the Galaxy. Its spectroscopic survey combining with the surveys in other wavebands, such as radio, infrared, X-ray and γ-ray will make important contribution to the cross-identification of multi-waveband of celestial objects. The large sample spectroscopic sky survey has been made dramatic progress in recent years, especially due to the success of 2dFand SDSS projects. With its powerful spectroscopic survey ability, LAMOST is expected to push it deeper and wider. To maximize the scientific potential of the facility, wide national participation and international collaboration have been emphasized. The survey has two major components: the LAMOST ExtraGAlactic Survey (LEGAS) and the LAMOST Experiment for Galactic Understanding and Exploration (LEGUE) survey of Milky Way stellar structure.