The conception of magmatic cycle is brought forward and the division of magmatic cycles is preliminarily studied in this pap er. Two magmatic cycles, named Huabei episode Ⅰand Huabei episode Ⅱ, have been identified in terms o f structural evolution, magmatism and petrochemistry for the Tertiary magnetism in the Bohai Bay basin. The first episode is composed of stages Ek—Es 4, Es3—Es2 and Es1—Ed, while the second one is composed of stage Ng—Nm. In every stage, the early magmatic activity is differe nt from the late one.