In the Western Hills, Beijing, Cambrian-Ordovician strata are well developed. Based on the characteristics of the rock association coupled with sedimentary structures and paleontological and geochemical indicators, 6 major types of parasequences may be distinguished. They were formed in different environments. Their thicknesses are commonly a few meters and, in a few cases, in the range from several dozens of centimeters to dozens of meters. Each type of parase-quence is composed of 4-6 kinds of rocks, but a single parasequence may be composed of only two kinds of rocks. Within each parasequence, the depth of water always becomes pregressively shallower upward, and between two parasequences, there always is a f acies jump from shallow to deep water. From the characteristics of these types of parasequences, it may be inferred that there is an abrupt deepening process of sea water between each two parasequences. And the presence or absence and stacking pattern of the parasequences depend on the abrupt deepening amplitude, the relation between the steady deepening and the deposition rate and the lasting time of the steady deepening. The system tract and sequence can be determined by the type of the parasequence group (regressive, progressive and accumulative), which is recognized from the trend of upward change of the same type of parasequence at the section. In addition, it is estimated that the abrupt deepening of sea water could not result from ice melt. So the Milankovich cycle could hardly lead to parasequence formation.