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In this paper, we report 591 high velocity star candidates (HiVelSCs)selected from over 10 million spectra of the data release seven (DR7) of theLarge Sky Area Multi-object Fiber Spectroscopic Telescope and the second Gaiadata release, with three-dimensional velocities in the Galactic rest-framelarger than 445 km/s. We show that at least 43 HiVelSCs are unbound to theGalaxy with escape probabilities larger than 50%, and this number decreases toeight if the possible parallax zero-point error is corrected. Most of theseHiVelSCs are metal-poor and slightly alpha-enhanced inner halo stars. Only 14%of them have [Fe/H] > -1, which may be the metal-rich "in situ" stars in haloformed in the initial collapse of the Milky Way or metal-rich stars formed inthe disk or bulge but kinematically heated. The low ratio of 14% implies thatthe bulk of stellar halo was formed from the accretion and tidal disruption ofsatellite galaxies. In addition, HiVelSCs on the retrograde orbits have aslightly lower metallicities on average compared with these on the progradeorbits, meanwhile metal-poor HiVelSCs with [Fe/H] < -1 have an even faster meanretrograde velocity compared with metal-rich HiVelSCs. To investigate theorigins of HiVelSCs, we perform orbit integrations and divide them into fourtypes, i.e., hypervelocity stars, hyper-runaway stars, runaway stars and fasthalo stars. A catalog for these 591 HiVelSCs, including radial velocities,atmospheric parameters, Gaia astrometric parameters, spatial positions, andvelocities, etc., is available athttp://paperdata.chinavo.org/LYB/lamostdr7_gaiadr2_hvs_591.csv.

591 high velocity stars in the Galactic halo selected from LAMOST DR7 and Gaia DR2