[Objective] To study the form of calcium carbonate and its effect on the loess microstructure in order to provide basis for the study of influence mechanism of calcium carbonate on the loess liquid and plasticity limits.[Methods] First, using distilled water to wash soluble salt in loess, and obtained loess samples with different contents of calcium carbonate processed by 1 mol/L acetic acid solution. Liquid and plastic limits joint meter was used to measure loess liquid plastic limit indicator. The scanning electron microscope (SEM) was used to analyze the X-ray energy dispersive microanlysis(EDX).[Results] ① Under the action of acetic acid, the liquid plastic limit decreased significantly with the decrease of calcium carbonate. ② Calcium carbonate blocks and superfine calcium carbonate were exposed as the skeleton and set granules to Si, Al and Si-Fe, and its main function was to connect the cementation between the particles and the pore skeleton of the loess.[Conclusion] In the early stage of acetic acid treatment, the calcium carbonate was mainly dissolved in the clay granule, and the loss of granule led to the loss of loess sand soil and eventually affected the liquid and plastic limit index.