The Yangmugan boron deposit is taken as the target on field observations, photomicrographs and isotopic characteristics to study the role of sedimentary formation played in the mineralization process of Paleoproterozoic boron deposit in Liaodong Peninsula. The Yangmugan boron deposit, which is a typical ascharite-type boron deposit within Jiao-Liao-Ji belt, has main ore bodies distributed in Li'eryu Formation and Gaojiayu Formation of Paleoproterozoic Liaohe Group. Detailed field and photomicrographs observations on different types and grades of ores review that palimpsest and metasomatic textures are main, reflecting a late metallogenic hydrothermal metasomatic process. Sulfur isotope reveals that the δ34S values of ascharite range by 14.2‰ to 19.5‰ with an average of 17.7‰; biotite granulite and tourmaline granulite have δ34S values from 12.0‰ to 13.3‰ with an average of 12.7‰; and serpentinized marble and serpentinite samples have δ34S value from 15.6‰ to 23.1‰ with an average of 19.3‰. Sulfur isotope reveals that boron is sourced from the sedimentary and volcanic formations of Liaohe Group. Taking previous studies on boron isotope into consideration, it is believed that the sedimentary formation also plays a key role in the deposition of Yangmugan boron deposit. Although boron deposit has a close relationship with the evolution of Paleoproterozoic Jiao-Liao-Ji belt, the active B may have been activated and redistributed in many geological processes, the early evolution of Jiao-Liao-Ji belt can not be constraint only by the distribution of boron deposit.