The Earth’s deep carbon cycle is of great significance for studying the global climate change, life exploration, and lithospheric evolution. The lithosphere beneath the eastern China is an important place for the deep carbon cycle, and its thinning and destruction are closely related to the deep carbon cycle. The subduction of the Pacific plate in the Mesozoic is the key to the thinning and destruction of the lithosphere in Eastern China, and plays a vital role in the large- scale metal mineralization in North China Craton and South China Block. This paper systematically elucidates the principle of Mg isotope tracing the Earth’s deep carbon cycle, gives an example of the deep carbon cycle in the eastern China, and discusses the interaction of the mantle with the carbonated melts/fluids released from the subducted paleo- Pacific plate during the Mesozoic, which is an important reason accounting for the commonly existed light Mg isotopic composition in the eastern China. Furthermore, the polysolvability of light magnesium isotopes is pointed out and the future trend of studying the Earth’s deep carbon cycle via multi- isotope joint tracing is also proposed.