Abstract: The saline-sodic mires with areas of 578000 hm² in the western Songnen Plain are one kind of the important land resources and the most prospective landscape.However, they faced to rapid development of salinisation and degradation of ecological function.The study of the formation and changes processes is significant for saline-sodic mires management.In this paper the mechanism of saline-sodic mire formation and development was analyzed systematically using the methodology and theory of environmental change.The genesis of the mire here are:① uneven subsidence of the crust caused by Neo-tectonic movement, ② widespread distributed paleo-river channels and water-logged lands caused by frequently river channel changes since Late Pleistocene, ③ wind erosion resulted in the eolian water-logged land mires, ④ paleo-climate also caused the mire changes.The processes of mire salinisation include historical relic salinisation and modern salinisation.The mechanism of historical relic salinisation is the long term accumulation of salts because of the evaporation in the closed lake basins formed after the disappearance of paleo-Songliao Big Lake, the logged lands formed by river channel changes and eolian erosion.In this area there was a lake as big as 50,000km² during early Pleistocene and middle Pleistocene.Since late Pleistocene this lake disappeared and formed lot of small closed shallow ponds or water-logged lands because of upward crust movement and sedimentation.For example in Dapushu Lake the total dissolved salt in sediments is as high as 6mg/kg, PH as high as 10.3.The paleo-river channel changes also played substantial role in mire salinisation.Since late Pleistocene about 40 big river channel changes occurred and formed large area of closed lakes.The modern process of mire salinisation is caused by the dwindling of fresh water supplement and damps of the drainage after construction of dams of the rivers and reservoirs.For example in low Taoerhe River reach, the water total salt content floodplain is two times of that in channel because of river dams.The drought and destroyed vegetation also accelerated this process.The effective measures of protection salinisation and mire management are planting paddies, irrigating reds using floods and setting up nature reserves.