The C and CO reduction proportional fraction was studied in this work to evaluate consumption of the reducer accurately. Based on two-step reaction mechanism (TSRM) theory, a phase equilibrium calculation model (PECM) was established. As three typical kinds of reducers, C, CO, and CH4 were introduced to the calculation. Reduction of C is the main reaction for the C reducer. Co-reduction of C and CO happens for the CO reducer. As a decomposition product of CH4, H-2 plays an important role of reduction, as half of the copper slag is reduced by H-2. Effects of temperature, reducer addition ratio, and Ca/Si ratio on the indirect reduction index (IDR) and reduction reaction proportional fraction are discussed. With an increase of C and CH4 reducer addition ratio, the reduction degree of C was strengthened. With addition of CaO, the CO reduction reaction and CH4 cracking reaction were all promoted. The accuracy of the calculated results was verified by a temperature-programmed direct reduction experiment. As a renewable reducer, biochar was introduced in verified experiments.