A platinum/palladium/rhodium (Pt/Pd/Rh) spent catalyst (Pt/Pd/Rh ratio of about 0.16/0.001/0.01 wt%) supported on MgO–Al2O3–SiO2 as the main carrier was studied in the methane partial oxidation process. By utilizing the characteristics of microwave selectivity to couple with the material with high dielectric losses, micron iron powder was chosen during the microwave treatment since it has a higher dielectric constant than the Pt/Pd/Rh components. It was placed at the bottom of the reactor, where it could absorb microwave energy and convert it into heat energy, thereby enabling the partial oxidation reaction to occur. Under the same experimental conditions, namely, CH4/air volume ratio of 1:2, microwave power of 450 W, and reaction time of 330 min, the results showed that the yields of syngas were higher when the micron iron was used with the Pt/Pd/Rh spent catalyst than when the catalyst was used alone. The yields were 67.3 and 11.6%, compared with 41.5 and 11.5%, for H2 and CO, respectively, when using the micron iron combined catalyst rather than micron iron alone. It was also found that the coke deposed on the surface of the Pt/Pd/Rh catalyst had a filamentous shape.