In this study, five farmland soils (black soil, light chernozem soil, fluvo-aquic soil, sierozem soil and Huangmian soil) with obvious different CaCO₃ contents were selected to analyze the distributions and correlations among soil organic carbon (SOC), calcium carbonate (CaCO₃) and calcium components within bulk soil and aggregate fractions, and evaluate the effect of CaCO₃ on organic carbon accumulation in alkaline dryland soil. The results showed that no significant correlation existed between SOC and CaCO₃ contents in bulk soil, but they were significantly negatively correlated in 0.002-0.053 mm and <0.002 mm aggregates (R² were 0.67 and 0.83, respectively). Excessive CaCO₃ affected the accumulation of organic carbon in micro-aggregates. The extractable and oxidizable calcium were the main calcium forms affecting the accumulation of organic carbon in microaggregates which accounted for 64.09% more of the total calcium content in soil, and their contents were increased with the decrease of aggregate size. Ca²⁺ is an important cement of organic-inorganic complexes, but excessive Ca²⁺ can seize organic carbon binding sites on soil particles and combine with clay and silt particles to form a micro-aggregate structure, which hinders the formation of organic-inorganic complexes and the accumulation of organic carbon in micro-aggregates.