Environmental changes during the Ordovician to Silurian transition (OST) and the cause of Late Ordovician Mass Extinctions (LOMEs) remain a subject of debate. This study presents the first continuous seawater pH record spanning the Late Ordovician and Early Silurian, based on carbonate boron isotope (delta 11Bcarb) data obtained from a carbonate-dominated section in South China. Our results reveal predominantly stable delta 11Bcarb values throughout the Late Ordovician and Early Silurian, punctuated by a positive delta 11Bcarb excursion during the Hirnantian coinciding with Gondwana glaciation. The calculated seawater pH pattern indicates a generally low pH baseline across the OST, temporarily interrupted by a transient increase in surface ocean pH coinciding with the glacial episode. These pH fluctuations are interpreted to result from a combination of factors, including declining atmospheric pCO2 levels, sea level changes, weathering of carbonate rocks, and decomposition of organic matter. This study suggests that the fluctuation of seawater pH exerted a crucial role in the climatic changes and biotic evolution during the OST. The enhanced carbonate weathering and increased seawater pH, together with sea level fall and a reduction in shelf area, likely contributed to the decreased net accumulation of carbonates and represented a negative feedback for the development of glaciation and cooling climate. Given that the living of organisms (e.g. brachiopod, conodont, sponge and radiolarian) was sensitive to the changes in seawater pH, if and how the seawater pH fluctuations affected the LOMEs still needs more detailed work in the future.