Wetlands have many important functions. To a wide range of wildlife species, they offer critically important habitats. They also act to mitigate flooding, regulate micro and macro climate changes, degrade pollutants and control erosion etc. Wetland benefits are these functions, which provide direct, indirect, and non-use values to humans. In this study, field soil data are used to calculate the flood mitigation benefits of wetland soils within the Momoge National Nature Reserve, Jilin Province, the People’s Republic of China. Calculations are based upon environmental economic assessment methods and GIS techniques. The estimated flood mitigation capacity of wetland soils within the Momoge Reserve was 7.15 × 104 m3/hm2/yr. This translated into an economic benefit of 5700 $/hm2/yr due to flood mitigation. Spatial differences in the flood mitigation ability of soils were observed across the Momoge wetlands. Benefits associated with flood mitigation were highest within the middle reaches of the Momoge wetlands and least in the East. This quantitative analysis of flood mitigation benefit, with its investigation of wetland soils, will be a useful reference both for the assessment of wetland values in the local region and also for the greater understanding wetland function and value assessment methods.