Previous studies have suggested that the human Y-chromosome haplogroup Q1a1a-M120, a widespread paternal lineage in East Asian populations, originated in South Siberia. However, much uncertainty remains regarding the origin, diversification, and expansion of this paternal lineage. To explore the origin and diffusion of paternal Q-M120 lineages in East Asia. The authors generated 26 new Y chromosome sequences of Q-M120 males and co-analysed 45 Y chromosome sequences of this haplogroup. A highly-revised phylogenetic tree of haplogroup Q-M120 with age estimates was reconstructed. Additionally, a comprehensive phylogeographic analysis of this lineage was performed including 15,007 samples from 440 populations in eastern Eurasia. An ancient connection of this lineage with populations in Siberia was revealed. However, this paternal lineage experienced an expansion between 5000 and 3000 years ago in northwestern China. Ancient populations with high frequencies of Q-M120 were involved in the formation of ancient Huaxia populations before 2000 years ago; this haplogroup eventually became one of the founding paternal lineages of modern Han populations. This study provides a clear pattern of the origin and diffusion process of haplogroup Q1a1a-M120, as well as the role of this paternal lineage during the formation of ancient Huaxia populations and modern Han populations.