Such characteristics of calcium ferrite as mass fraction，crystal morphology and crystal size have an important influence on the quality of sinter. The structure of on-site sintering blending is used as the benchmark，the micro-sintering experiments with different temperature gradients are carried out by using analytical reagent，the formation characteristics of calcium ferrite during the sintering process are analyzed emphatically by using polarized light microscopy，and its influence law on the metallurgical properties of sinter is analyzed. The results show that during the temperature rising from 1280 ℃ to 1400 ℃，the sinter is dominated by porphyritic structure，in which the mass fraction of different forms of calcium ferrite decreases by 26.40%，and the RDI_+3.15mm and RI index of sinter decrease from 74% and 72% to 69.20% and 67.57% respectively. During the heating process of 1250~1280 ℃ and the cooling process of 1400~1200 ℃，the sinter is dominated by interwoven molten structures，while a large number of acicular and columnar calcium ferrite is produced，the Al₂O₃/SiO₂ in acicular and columnar calcium ferrite is smaller，which is 0.25，while the RDI_+3.15mm and RI index of sinter are getting better. During the cooling process from 1200 ℃ to room temperature，the main structure of sinter is corrosion structure，which is the formation stage of prismatic tabular and heteromorphic calcium ferrite. Especially in the cooling stage from 1200 ℃ to 1100 ℃，the ratio of acicular and columnar calcium ferrite is 32.70%，while the RDI_+3.15mm and RI index of sinter are relatively high，reaching 79.20% and 80.76% respectively.