We identify member stars of more than 90 open clusters in the LAMOST survey. With the method of Fang et al.(2018), the chromospheric activity (CA) indices logRCaK for 1091 member stars in 82 open clusters and logRH{alpha} for 1118 member stars in 83 open clusters are calculated. The relations between the average logRCaK, logRH{alpha} in each open cluster and its age are investigated in different Teff and [Fe/H] ranges. We find that CA starts to decrease slowly from logt = 6.70 to logt = 8.50, and then decreases rapidly until logt = 9.53. The trend becomes clearer for cooler stars. The quadratic functions between logR and logt with 4000K < Teff < 5500K are constructed, which can be used to roughly estimate ages of field stars with accuracy about 40% for logRCaK and 60% for logRH{alpha}.
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