The combined application of microwave energy and special catalysts holds the potential for the selective conversion of biomass during the pyrolysis process. In this work, steel slag (SS) and biochar-based Fe (RHC/Fe) were adopted as both low-cost Fe-based catalysts and microwave absorbers in the microwave-heating pyrolysis system for the upgradation of bio-oil from macroalgae (Sargassum) and lignocellulosic biomass (pine sawdust). The catalysts all showed strong microwave adsorption performance, exhibiting higher heating rates than that of SiC. SS with a high content of iron oxides plays a catalytic role in biomass decomposition, while RHC/Fe with a highly porous structure not only promotes biomass decomposition but also enhances the secondary reaction of volatiles. The presence of SS significantly improved the content of lighter compounds (<C10 compounds) in Sargassum-derived bio-oil, leading to an improvement in the yield of potential chemicals such as phenols and furans; the formation of phenols was also significantly enhanced by SS during pine sawdust pyrolysis with a high phenols content of 55.8% in the obtained bio-oil. RHC/Fe exhibited remarkable selectivity on Sargassum pyrolysis and the obtained bio-oil contained only 13 compounds including 33.8% of <C10 compounds and 66.2% of >C20 compounds without any C10–C20 compounds. The N-containing compounds in Sargassum-derived bio-oil were successfully removed using RHC/Fe. RHC/Fe also showed a significant catalytic effect on the production of combustible gases from the pyrolysis of Sargassum and pine sawdust, with significant improvement in the total gas yields and combustible gas concentrations.