By combining landscape ecology with GIS spatial analysis, this paper analyses the dynamics of the marsh landscape structure of the Sanjiang Plain in the past 30 years. Furthermore, taking Fujin County, located in the north of the plain, as an example, we analyze the conversion between marsh and other land use types. The marsh in the Sanjiang Plain decreased greatly in the past 30 years, but the trend has begun to diminish. The marsh area decreased by 51.33% from 1980 to 1996, whereas it decreased by 4.19% from 1996 to 2009. The fragmentation of the marsh increased; the number of the patches increased by 326 from 1986 to 1996, whereas it only increased by 18 patches from 1996 to 2009. It is obvious that the speed of patch development diminished and the marsh fragmentation decreased, which shows that the reclamation of the marsh converted from the fragmentation of central area to the perimeter in large area of the marsh. The reclaimed marsh has been converted mainly to paddy field and dry land. Large-scale reclamation in the Sanjiang Plain influences its natural environment directly: the climate of the region turns from cold and wet to warm and dry, which makes the marsh both in the low-temperature north part and in the deeply stagnant east part suitable for further agriculture development. It is coincidental with the offset to southwest of the centroid of the marsh distribution in the Sanjiang Plain. The mechanism of marsh dynamics in the Sanjiang Plain is analyzed here. The marsh loss is mainly caused by human activities. It is impossible for the marsh in the Sanjiang Plain to naturally recover itself. Therefore, the restoration of the former marsh must rely on humans.