  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
logR'CaK for 1091 member stars in 82 open clusters and logR'H{\alpha} for 1118
member stars in 83 open clusters are calculated. The relations between the
average logR'CaK, logR'H{\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 logR'CaK and 60% for logR'H{\alpha}.
