Abstract: The erosion of the blast furnace hearth lining by harmful elements is a key factor limiting the life of the blast furnace. This study systematically investigates the differential erosion mechanisms of ultramicroporous carbon bricks by Na, K, and Zn elements through accelerated high-temperature tests and proposes a novel erosion index (EI) model. Microscopic analysis revealed that Na and K reacted with carbon brick components to generate low-melting point compounds, which accelerated interfacial melting loss and carbon loss, whereas Zn existed in the form of simple substances or oxides, which led mainly to the expansion of carbon bricks through physical penetration. The erosion index model, developed based on experimental data, quantitatively illustrates the erosive intensity of various elements on carbon bricks (the erosion index of Na exceeds that of K, which in turn is higher than that of Zn and blast furnace slag-iron), which provides a theoretical basis for the selection of refractory materials for blast furnaces and the control of harmful elements. This study advances understanding of microstructural degradation mechanisms and provides practical guidance for extending blast furnace campaign life.