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The first Water Cherenkov detector of the LHAASO experiment (WCDA-1) has beenoperating since April, 2019. The first 10 months of data have been analyzed totest its performance by observing the Crab Nebula as a standard candle. TheWCDA-1 achieves the sensitivity of 65 mCU per year with a statistical thresholdof 5 $\sigma$. In order to do so, 97.7\% cosmic ray background rejection ratearound 1 TeV and 99.8\% around 6 TeV with an approximately photon acceptanceabout 50\% by using the $compactness$ of the shower footprints to be greaterthan 10 as the discriminator between gamma induced showers and the cosmic raybackgrounds. The angular resolution is measured using the Crab Nebula as apoint source about 0.45$^\circ$ at 1 TeV and better than 0.2$^\circ$ above 6TeV with the pointing accuracy better than 0.05$^\circ$. They are all matchingthe design specifications. The energy resolution is found 33\% for gamma raysaround 6 TeV. The spectral energy distribution of the Crab Nebula in the rangefrom 500 GeV and 15.8 TeV is measured and in agreement with results of otherTeV gamma ray observatories.

Performance of LHAASO-WCDA and Observation of Crab Nebula as a Standard Candle