<p>The life of the blast furnace hearth directly determines the overall life of the blast furnace, and the thickness of the hearth lining is an important parameter affecting the safe operation of the blast furnace. The thermocouple method currently used for hearth thickness monitoring is severely constrained by the arrangement and longevity of the couples. For the impact echo method applied in the thickness detection of the furnace hearth lining, there are problems that it is difficult to calibrate the wave velocity of multi-layer materials and the detection accuracy is low, two multi-layer media equivalent wave velocity formulas are proposed, and by combining it with the impact echo method, two impact echo equation methods for detecting the thickness of the blast furnace hearth are proposed. The two-dimensional layered numerical model is established for theoretical validation and research on the applicability range of the two methods. And the complex model is established to test the method according to the shape of the furnace hearth liner after severe erosion. Finally, blast furnace test validation is realized. The results show that the proposed equivalent wave velocity formula and impact echo equation method can be better applied to safety monitoring of blast furnace hearth thickness.</p>