With the spread of historical materialism, Marxist theory of social formation has been introduced into the research area of ancient history in China. Guo Moruo (郭沫若) is the standard bearer in it. Influenced by Morgan and Engles, he associated closely the development level of iron tools with the changes of ancient society in China. Therefore, he devoted himself to finding the iron in the history of China. Through his exploration in philology, ancient artifacts and socio-economic history, he took Shi Ya (《石雅》) by a modern epigrapher Zhang Hongzhao (章鸿钊), as an important reference, and developed an epistemic cognition of the evolution history of ancient iron in China. Then, taking the iron craft level as the main yardstick, he examined the economic development level of ancient society and identified the social formations of China in the pre-Qin period. Although the later Marxist historians’ theories of social formation research have their own merits, most of them are deeply influenced by Guo Moruo’s social research model. In a word, Guo Moruo pushed the application of historical materialism in China to a new height, created a new paradigm linking economic history with social formations, leading to the rise of Marxist historiography in China. At the same time, however, he relied too much on “iron” to judge the level of productive forces, and was thus prone to miscalculation or even fell into the mire of “determinism of production tools,” which became one of the crucial points criticized and attacked by other historians.