With the combination of GIS technology and remotely sensed data,cultivated land changing trend in regional scale can be easily traced and inventoried in time.Six periods of land use spatial data bases were established with remote sensing imagery data and scanned topographic map in ARC/Info 9.0 software.The cultivated land changing feature and mutual conversion with other land use were analyzed quantitatively.It was found that cultivated land drastically changed in the past 50 years.The cultivated land was only 17 133.8 km2 in 1954,the proportion of cultivated land only amounted to 15.91% in Sanjiang Plain;while the cultivated land amounted to 51.17% in 2005,which was 55 688.45 km2 in total area.Cultivated land increased 38 554.64 km2 in the whole study periods,and the average increased area is 755.97 km2 per year.By mutual conversion analysis of cultivated land and other land use,it was found that reclamation of wetland was the main contributor for cultivated land increase,and forest and grassland converted to cultivated land was another reason for its increasing.Cultivated ratio and changing rate were applied to analysis the cropland dynamics characteristics in the past 50 years,it was found that cultivated land changing area and rate showed significant variation during different study periods.Population increasing,large amount of immigrant swarming into Sanjiang Plain as result of national policy,enhanced by agricultural technology development was the main reason for the cultivated land increase in the whole study periods.Still,it should be notice that natural conditions also played an important role in the cultivated land changing trend,especially,topographic and soil environment cast an impact on cultivated land changing location,and its pattern as well.