Abstract: Based on petrological and geochemical analysis of middle-upper Cambrian dolomite in three different profiles, the authors hold that the dolomite was mainly formed by brine reflux dolomitization in the early bury period, together with mixing water replacement. The evidence obtained is as follows: ① the ratios of Sr/Ba in dolomite and calcite are high, with only a few data being lower than 1; ② the values of Na and K are high in calcite, the content of Na in calcite is higher than that of middle Ordovician Mifflm Member calcite in Wisconsin and that of upper-Ordovician calcite in Tongling of eastern Tennessee characterized by fresh water origin, and the content of Na in dolomite is equal to that of Sabkha dolomite in Barbwire terrace of Canning basin; ③ the values of m suggest that the calcite was in a seawater sedimentary environment, and the dolomite was in a lagoon sedimentary environment; ④ the values of revised B show that the calcite and dolomite were formed in salt and semi-salt water, partly affected by fresh water. The Sr values were impoverished during the dolomitization, and Na and K might also have been impoverished in this process.