Abstract: Sandy dolomite has special physical and mechanical properties such as a fragmented rock structure and low strength. The geological characteristics and formation mechanisms of sandy dolomite vary under different layers and geological structures. To reveal the macro and micro formation mechanisms of sandy dolomite in the Hewanjie Formation (T₂h) of the Triassic in western Yunnan, this paper analyzes and summarizes the spatial distribution characteristics of sandy dolomite in the study area. At the same time, different grades of sandy dolomite rock samples were selected for electron microscopy scanning and energy spectrum analysis experiments to explore and analyze their macro and micro formation mechanisms. The research results indicate that: (1) T₂h sandy dolomite in western Yunnan exhibits vertical zoning in spatial distribution, forming fractures and structural planes under the action of tectonic stress, providing conditions for dissolution; (2) According to electron microscopy scanning experiments, the micro dissolution mechanism of sandy dolomite is analyzed as follows: dissolution along the crystal surface and cracks-crystal disintegration and detachment-continuous expansion of dissolution pores and fractures. At the same time, it can be explained that dissolution plays a dominant role in the micro formation mechanism of T₂h sandy dolomite in western Yunnan; (3) Through chemical composition analysis, it was found that during the evolution process from slightly sandy dolomite to completely sandy dolomite, the Mg element continuously migrated and was lost at a relatively fast rate.