The larger the difference between the willingness scale of tobacco family farmers and the optimal scale of efficiency, the greater the degree of irrationality, and the higher the decision making risk. With the aid of DEA model, this study calculated the optimal scale of efficiency of Guiyang tobacco family farms. Using the ratio of willingness scale and efficiency optimal scale, it measured the degree of irrationality of family farmers. In addition, with the help of multiple linear regression model, it explained the irrational decision making mechanism of family farmers. Finally, it made a portrait of farmers who tend to make irrational decisions, to find specific farmers and guide them in their production and operation, reduce the risk of planting scale decision making and stabilize the sustainable development of the tobacco industry.