Coastal wetlands are the last barriers for pollutants from land to the sea. In this study, a coastal wetland that locates in the lower reach of Haihe River Systems was selected to speculate the removal and retention of polycyclic aromatic hydrocarbons (PAHs) by analyzing their spatial distributions and the changes of composition. The results showed that the overall removal efficiency of PAHs in water phase was 58.1%. There was an accumulation for sedimentary PAHs, reaching 431 ng/g (181 ng/g in the inlet). The compositions of sedimentary PAHs were also changed, high-molecular-weight PAHs were the main component (70−50%), with a steady decreasing trend and the influence of water flow direction. The risk assessment by mean effect range media quotients (M-ERM-Qs) depicted that there was in low ecological risk, due to the degradation of PAHs in the wetlands. Our results clearly demonstrated the coastal wetlands could effectively retain the PAHs, thus we recommend an active protection strategy for the coast wetlands in Tianjin in the future.