Abstract: Under the "double carbon" targets in the new era, China's iron and steel industry has entered a critical stage of green development. The high-proportion pellet technology, which uses basic pellets to replace sintered ore, is one of the important technical directions for low-carbon blast furnace smelting in China. However, basic pellets have a major problem of abnormal swelling during the reduction process, which can cause major production accidents such as abnormal furnace conditions. To address this issue, this study conducted reduction swelling rate tests on pellets with natural basicity and basicity ranging from 0.2 to 1.2. Results show that the swelling rate of pellets increases with the increase of basicity, reaches the maximum value when the basicity is 0.8, and decreases significantly as the basicity further increases. X-ray diffraction (XRD) analysis and optical microscopy were used to analyze the mineral phase changes of pellets with basicity of 0.8 and 1.2 during the reduction process. Results indicate that the main binding phase of pellets with basicity of 0.8 is calcium iron silicate minerals, which have poor reducibility and a lower reduction rate than hematite. The inconsistent internal stress generated between the two during reduction leads to abnormal swelling of the pellets. In contrast, the main binding phase of pellets with basicity of 1.2 is calcium ferrite (SFCA), whose reduction rate is basically consistent with that of hematite. The internal stress generated between the two during reduction is basically uniform, thus avoiding abnormal swelling. Therefore, it is recommended that pellets with a basicity of more than 1.0 be produced in industrial production to reduce the swelling rate of pellets.