Both climate and cultivated land area of Puan County, Guizhou Province, China are briefly described. The six steps of Gray Series Forecasting Model are introduced, including generation of series, generation of mean value, establishment of GM (1,1) model, reducing reaction, reliability test, and extrapolation forecast. According to the data of cultivated land area in Puan County from the year 1998 to 2007, Gray System Theory is used to establish the Gray Series Forecasting Model of cultivated land area in Puan City. Since it has passed the reliability test, this model has relatively high fitting accuracy and can be used for extrapolation forecast. Based on this Gray Series Forecasting Model, extrapolation forecast of cultivated land change is conducted in Puan County from 2009 to 2012. Result shows that the expected area of cultivated land will be reduced by 1050 hectares in the years 2009-2012, an annual decrease of 270 hectares. Cultivated land area shows a declining trend year by year. Result of the forecasting model is close to actual value with small prediction error. Thus, it can be concluded that the result is basically credible.