The Sanjiang Plain saw a significant decrease in marshland and increase in cropland from 1954 to 2005 due to human activities. Remote sensing and geographic information system technology were used to analyze this land conversion process. The study found that marsh area decreased by over 75% while cropland increased by 2.3 times during this period. By 2005, cropland covered over half of the total land area in the region. The main factors driving this conversion were population growth, climate warming, and national policies. The process of land conversion from marsh into cropland was largely completed by 2005, with large patches of marsh remaining only in some nature reserves.