Abstract: The dolomite reservoir in the 3^rd member of the Ordovician Yingshan Formation (Ying 3 member) in Gucheng area in Tarim Basin experienced multiple stages of transformation by diagenetic fluids during its evolution process, showing strong reservoir heterogeneity. There are great disputes about the dolomitization pathway and pore evolution process of dolomites with different occurrence. Accurately determining the key diagenesis period is very crucial to identify the relative sequence of diagenesis and its coupling relationship with reservoir pores in the evolution process of Ying 3 member dolomite reservoir. Based on techniques such as laser in-situ U-Pb isotope dating and element area scanning, the systematic laser in situ U-Pb dating is conducted on rock components of four core samples, including the matrix of host rocks (powder crystal dolomite, medium-fine crystal dolomite), medium-coarse crystal dolomite on the wall of vugs, and guest mineral in vugs (calcite and dolomitized vadose siltstone). The study results show that the high energy beach carbonates of Ying 3 member were primarily deposited at a geologic age of approximately 475.35 Ma in Gucheng area in Tarim Basin, laying a material basis for the development of reservoir. The reservoir pore evolution experienced two stages of dolomiti zation and one stage of calcite filling. The first stage of dolomitization occurred at 470.1 Ma, causing dolomitization of the fine-medium crystal dolomite in host rocks and the local vadose siltstones. The second stage of dolomitization was approximately at 452.1 Ma, leading to the further enlargement and coarsening of dolomite crystals of host rocks and the formation of medium-coarse crystal dolomite on the wall of vugs. The filling of calcite fluid was no later than 448 Ma, which damaged the local reservoir space. The main reservoir space of dolomite was inherited from the pre-existing pore space and adjusted by fault-hydrothermal transformation, rather than the traditional understanding that the pores were formed by the tectonic-hydrothermal corrosion. The above understanding provides basis for the study of pore genesis and prediction of effective dolomite reservoir in deep formation in Tarim Basin.