The Large High Altitude Air Shower Observatory (LHAASO) plans to build a hybrid extensive air shower (EAS) array with an area of about 1 km2 at an altitude of 4410 m a.s.l. in Sichuan province, China, aiming for very high energy gamma ray astronomy and cosmic ray physics around the spectrum knees. With an extensive air shower array covering an area of 1.3 km2 equipped with >40,000 m2 muon detectors and a 78,000 m2 water Cherenkov detector array, a sensitivity of about 1% Crab unit to gamma ray sources is achieved. Thus LHAASO will survey the entire northern sky for gamma ray sources with a full duty cycle and high sensitivity. The spectra of all sources in its field of view will be measured simultaneously over a wide energy range from 300 GeV to 1 PeV. This measurement will offer a great opportunity for identifying cosmic ray origins among the sources. The LHAASO is also equipped with 12 Cherenkov/fluorescence telescopes, so it will serve as an effective detector for energy spectrum measurements of different mass groups of cosmic rays.