We propose to study the water required by forestland in the north area of China, which includes the evapotranpiration and soil moisture contents. It is found that when the soil moisture content is below 45% of field capacity of soil and actual evapotranspiration is below 60% of potential evapotranspiration, the growth of trees cannot be sustained. On the basis of our research, we establish the computing method of minimum ecological water requirement of forestland, and take the 45% of field capacity of soil as minimum soil moisture quota of forestland, 60% of potential evapotranspiration as the minimum evapotranspiration quota. The minimum ecological water requirement of forestland of Huang huai hai area is estimated as an example on the basis of the rational area of forestland planned with GIS. The results indicate that the rational area of forestland of Huang huai hai area is 78 thousand km 2, proximately the 18.85% of total area, and the minimum water requirement is about 60.7 billion m 3, 48.3 billion m 3 for evapotranspiration and 12.5 billion m 3 for soil moisture contents.