Abstract: This paper analyzes and discussed the diagenetic mechanism for M5?5sub?member of Majiagou formation in Ordos basin by geochemistry approaches. The M5?5sub?member in Guanjiaya section is composed of powder crystal dolomite, calcareous dolomite and dolomitic limestone, belonging to a complete sedimentary cycle where the content, particle size and degree of order are gradually reduced from top to bottom. The power crystal dolomite’s carbon?oxygen isotope ratio is obviously negative, theδ18 O of calcareous and limestone matrix is partially negative, but theirδ13C values are close to the Ordovician water, and the87Sr/86Sr ratio is significantly increased. The results show that the original sedimentary sequence of this sub?member is characterized by micrite, packstone, disturbed biomicrite, calcarenite, and dolomicrite. With the loss of thee sea level, evaporated and concentrated seawater started various dolomitizations in turn from supratidal zone to subtidal zone until shallow burial environment. The powder crystal dolomite and dolomitic limestone originate in the reflux percolation of concentrated seawater