The first Water Cherenkov detector of the LHAASO experiment (WCDA-1) has been operating since April 2019. The data for the first year have been analyzed to test its performance by observing the Crab Nebula as a standard candle. The WCDA-1 achieves a sensitivity of 65 mCU per year, with a statistical threshold of 5 . To accomplish this, a 97.7% cosmic-ray background rejection rate around 1 TeV and 99.8% around 6 TeV with an approximate photon acceptance of 50% is achieved after applying an algorithm to separate gamma-induced showers. The angular resolution is measured using the Crab Nebula as a point source to be approximately 0.45° at 1 TeV and better than 0.2° above 6 TeV, with a pointing accuracy better than 0.05°. These values all match the design specifications. The energy resolution is found to be 33% for gamma rays around 6 TeV. The spectral energy distribution of the Crab Nebula in the range from 500 GeV to 15.8 TeV is measured and found to be in agreement with the results from other TeV gamma ray observatories.

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The Large High Altitude Air Shower Observatory (LHAASO) plans to build a hybrid extensive air shower (EAS) array with an area of about 1 km 2 at an altitude of 4410 m a.s.l. in Sichuan province, China, aiming at 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 km 2 equipped with >40,000 m 2 muon detectors and 78,000 m 2 water Cherenkov detector array, a sensitivity of about 1% Crab unit to gamma ray sources is achieved at 2×10 12 eV and 5×10 13 eV, thus the LHAASO will survey the entire northern sky for gamma ray sources with 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 from 10 11 eV to 10 15 eV. 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 measurement of different mass groups of cosmic rays over a wide energy range from 10 14 eV to 10 18 eV. Civil construction started in the middle of 2016, two years later, detector deployment began. The first water pool with 900 detector units covering an area of 22,500 m 2 was put into operation in April of 2019. 1200 scintillation detectors, 300 muon detectors and 6 telescopes were deployed afterwards. 1/4 of the LHAASO array will be in operation soon. Preliminary results of the LHAASO experiment are presented.