Analyzing a mill’s ability of flatness control has two purposes, one is finding the feather of a mill, and the other is making a judgment that whether a flatness error can be eliminated. But using classical method is hard to form a correct judgment when the error is complicated. Based on applying efficiency function in finding the property of each flatness actuator, a new method is developed to describe a mill’s comprehensive ability of flatness control. And we found the factors which contribute to whether a mill can weaken a certain flatness error. Practical application shows the new method is feasible.