The Middle Ordovician Majiagou Formation is in unconformable contact with both t he underlying and the overlying strata. It is a carbonate_evaporated tidal sequ ence deposited on a large shallow water carbonate platform. Cycles formed by suc h rocks as limestone, dolomite, gypsum or such sedimentary structures as birdeye structure, cracks,pseudocrystals of gypsum and halite are obvious. 84 cycles were recognized in the field and Fischer plots were used to analyse the cyclics equence. 3 third-order and 12 fourth_order sea level curves were differentiated. The 84 cycles are considered to be of a kind of Milankovitch rhythms formed by the eccentric variation of the earth. The Fischer plots are proved to be and effective tool for analysing the cycles formed in tidal environment.