Coastal salt marsh ecosystems are among the most important blue carbon (C) sinks. However, due to disparate data sources, limited sample sizes, and inconsistent methodologies, large variations exist in blue C stock estimation. Furthermore, studies focus on soil organic carbon (SOC), with minimal attention given to soil inorganic carbon (SIC). We conducted an intensive field survey across 114 sites along approximately 5000 km of China's coastline, investigating all C stock sectors associated with Phragmites australis, Spartina alterniflora, Suaeda salsa, and mudflats to better explore the biogeographical patterns and drivers of SOC and SIC and to estimate regional and national C pools. The results revealed significant spatial heterogeneity in SOC and SIC densities in the top 100 cm of soil, ranging from 20.89 to 246.95 Mg ha−1 and 0 to 249.33 Mg ha−1, respectively. These densities varied as a function of wetland location, vegetation type, climatic zone, and soil depth. The SOC and SIC content were jointly dependent on soil properties, climatic factors, and vegetation types. Soil available silicon was the most important environmental variable controlling both SOC and SIC. Climatic factors and vegetation types mainly controlled SOC and SIC through their interaction with soil properties. Increased vegetation productivity increased the SOC pool but decreased the SIC pool. Therefore, vegetation restoration in coastal wetlands should target suitable areas, such as soil pH < 7 or without existing SIC. Based on field measurements and published data, we estimated that the total C pool in the top 100 cm of soil across China's coastal salt marshes to be 91.55 Tg C, comprising 56.67 Tg SOC and 34.88 Tg SIC. These updated estimates, based on direct field measurements, provide new insights into blue C storage mechanisms and are crucial for validating parameterization of global coastal wetland C models.