Principle of AFC（Automatic Flatness Control） in single stand four-high inreversible cold foil mill is thoroughly studied. The whole process of flatness error， mechanical actuators corrections with multivariable optimal   method and calculation of spray level according to residual flatness error are presented. Practical application shows   that the average foil flatness error is well controlled in the scope of 0～3 I in stable conditions， which satisfies the   need of high precision control foil flatness.