Abstract: Carbon dioxide is likely the major greenhouse gas in the coastal wetland as the availability of sulfate inhibits CH₄ release. Therefore, the study of the CO₂ emission mechanism in the eco-system of the coastal wetland is of significance to carbon cycle and its budget. For this purpose, the LI-8100A soil respiration system was adopted to measure the soil CO₂ flux in the Liaohe Delta during May, 2012, and the method of enclosure chamber measurement was used to explore wetland CO₂ emission mechanism for three types of coastal wetlands: unvegetated wetlands, Suaeda salsa and Phragmites australis wetlands. It discovered that during this season of a year the Phragmites wetland has a higher respiration rate(3 041.59±320.66 mg·m^-2·h^-1)than the Suaeda wetland(534.09±56.06 mg·m^-2·h^-1), and both are higher than relevant records of CO₂ flux from literatures. Aboveground biomass has a positive liner correlation with the plant respiration rate. In the wetland ecosystem, CO₂ released from the soil accounts for ~60% of total CO₂ emission. It is also observed that the CO₂ emission will be significantly increased after wetland vegetation being destroyed, that means that the harvest of wetland plants is the cause of significant release of greenhouse gas CO₂.