It is an effective countermeasure to utilize biomass/biochar in blast furnace (BF) for CO2 emission reduction. In the present research, the biochar composite briquette (BCB) for BF was prepared using cold briquetting followed by heat treatment. The BCB is composed of 11.1 wt.% carbon, 72.7 wt.% magnetite, 11.25 wt.% wustite, 0.77 wt.% metallic iron, and 4.67 wt. % gangue. A BCB reaction model in BF was developed and was validated by experiments. Using the model, the BCB reaction behavior under simulated BF conditions was analyzed, and the BCB reaction behavior in an actual BF was predicted. Results showed that, under simulated BF conditions, the BCB underwent structure changes from 60 min (973 K) to 120 min (1273 K), forming low-melting compounds and transforming from the slag matrix to the iron network. In actual BF, the BCB reaction behavior is mainly divided into three stages including reduction by BF gas (473-853 K), reduction by BF gas, and partial self-reduction (853-953 K), and full self-reduction (953-1150 K). In the stages involving BCB full self-reduction, the BCB has the functions of increasing BF top gas utilization efficiency and lowering the temperature level of the thermal reserve zone.