China aims for net-zero carbon emissions by 2060, and an emissions peak before 2030. This will reduce its consumption of coal for power generation and steel making. Simultaneously, China aims for improved energy security, primarily with expanded domestic coal production and transport infrastructure. Here, we analyze effects of both these pressures on seaborne coal imports, with a purpose-built model of China’s coal production, transport, and consumption system with installation-level geospatial and technical detail. This represents a 1000-fold increase in granularity versus earlier models, allowing representation of aspects that have previously been obscured. We find that reduced Chinese coal consumption affects seaborne imports much more strongly than domestic supply. Recent expansions of rail and port capacity, which reduce costs of getting domestic coal to Southern coastal provinces, will further reduce demand for seaborne thermal coal and amplify the effect of decarbonisation on coal imports. Seaborne coking coal imports are also likely to fall, because of expanded supply of cheap and high quality coking coal from neighbouring Mongolia.