Abstract: It is prominent phenomenon of the conversion of dryland to paddy in the conditional areas because of high benefit on rice cultivation. The areas where the paddy field grows rapidly can lead to the contradiction between human and land. How to determine the maximum area of paddy field and to guarantee the harmonious development between human and land is a urgent problem to be solved. Based on the technology of GIS, taking Tonghe county, Heilogjiang province as research area, considering the nature constraints of the agricultural ecological bearing capacity, this paper calculated the maximum paddy area of permission in Tonghe county by quantified model. The results showed that the permission conversion degree should be controlled less than 0.45, that is, the paddy field area should be controlled within 79311.8 hm2. This limit can guarantee the harmonious development between human and land in Tonghe county. Based on the agricultural circumstances of 2008, the area of dryland converting to paddy field in Tonghe county is about 20 000 hm2 in the future and there is a big potential of dry land to paddy field. The research can provide a quantitative basis for agricultural planting structure adjustment in Tonghe county, Heilongjiang province and ensure the sustainable development of agriculture in Tonghe county and the similar regions.