We identify member stars of more than 90 open clusters in the LAMOST survey. With the method of Fang et al., the chromospheric activity (CA) indices, log R-CaK ' for 1091 member stars in 82 open clusters and log R-H alpha' for 1118 member stars in 83 open clusters, are calculated. The relations between the average log R-CaK', log R-H alpha' in each open cluster and its age are investigated in different T-eff and [Fe/H] ranges. We find that CA starts to decrease slowly from log t = 6.70 to log t = 8.50, and then decreases rapidly until log t = 9.53. The trend becomes clearer for cooler stars. The quadratic functions between log R' and log t with 4000 K < T-eff < 5500 K are constructed, which can be used to roughly estimate ages of field stars with accuracy about 40% for log R-CaK'and 60% for log R-H alpha'.