Abstract: To clarify the properties of diagenetic fluid, the evolution of sedimentary environment, the source of Mg-rich fluids, the migration paths of fluids is important to explain the genesis of dolomite and understand the formation process of dolomite. This paper is a case study of the Middle-Upper Cambrian Xixiangchi Formation in central Sichuan Basin. Based on the analyses of petrological characteristics, sedimentary cycles, trace elements as well as carbon, oxygen and magnesium isotopes, this paper systematically investigates the changes of sedimentary-diagenetic environment and the migration rule of Mg-rich fluids, and explores the genesis of dolomite in Xixiangchi Formation. The dolomitization fluid of Xixiangchi Formation is dominated by Cambrian contemporaneous seawater, and dolomitization occurred in the penecontemporaneous-shallow burial stage. The Xixiangchi Formation in central Sichuan Basin is generally dominated by shallow water, brackish water and saline water marine sediments under the warm and arid subtropical climate. Sabkha dolomitization and seepage-reflux dolomitization are dominant. There is a coupling relationship between the variation trend of δ²⁶Mg value of dolomite in Xixiangchi Formation and the sedimentary cycle. When Mg-rich seawater migrates vertically, near-source seepage-reflux dolomitization occurs, and the δ²⁶Mg of dolomite gradually increases downward; when Mg-rich seawater migrates laterally, far-source seepage-reflux dolomitization occurs, and the δ²⁶Mg of dolomite does not vary vertically significantly; Sabkha dolomitization mainly occurs in an evaporation and restriction environment, and the δ²⁶Mg of dolomite gradually decreases downward. The formation of dolomite in Xixiangchi Formation is mainly controlled by sea level changes. Thin-bedded dolomite can be formed in the low sea level period, and large-scale thick-bedded dolomite reservoir can be formed by layer-by-layer superimposition of multiple dolomitization in the high sea level period.