Carbon-14 (14C) has a relatively high concentration factor in organisms, and its distribution and migration patterns in marine environments have attracted much attention. In May 2023, samples were collected from the tidal flats of the Changjiang Estuary to investigate the concentration distribution and migration patterns of 14C in water, sediment and organisms. The results indicate that the 14C activity concentration in seawater is 4.95–9.71 mBq/L (mean: 6.54 ± 0.37), which is comparable to that in other marine regions. The sediment 14C activity concentration is 0.23–0.76 Bq/kg (mean: 0.50 ± 0.04), which is related to the complex regulation of the carbon cycle by hydrodynamic and biological disturbances. The 14C concentration in organisms is 9.50–24.8 Bq/kg·fresh weight (mean: 17.9 ± 1.60), which is affected by feeding habits. The Sediment–Water Distribution Coefficient (Kd = 37–114 L/kg) and the concentration factor (CF = 1243–4472 L/kg) are significantly lower than the values recommended by the International Atomic Energy Agency (IAEA). This suggests that optimizing the migration model for 14C and establishing localized databases for Kd and CF values is crucial. Radiation dose assessment reveals that the annual effective dose for residents ranges from 1.43 to 3.74 × 104 mSv, suggesting very little radiological health risk under current environmental conditions. This study provides baseline data on 14C in Chinese intertidal systems for the first time, offering a scientific basis for environmental monitoring and ecological risk management of nuclear power plants.