Abstract: From the century discovery of cosmic ray, the origin of cosmic ray is still a mystery. Study on the origin of high energy cosmic ray is in a cross field between the very high energy (VHE) gamma ray astronomy and cosmic ray physics. The Large High Altitude Air Shower Observatory (LHAASO) is a unique and new generation experiment with the advantages with an high altitude and a large effective area. It takes function of hybrid technology to detect cosmic ray, improving the separating power between gamma ray and cosmic rays. LHAASO is expected to survey full sky to find new gamma ray sources, to obtain the highest sensitivity to > 30 TeV gamma ray detection, and to make very high precision measurement in cosmic rays spectrum in energy range of 5 orders. LHAASO will provide a decisive judgement for unraveling the mystery of the origin of cosmic ray. This paper describes the structure, performances, and scientific motivation of LHAASO.