Mathematic essence of the highest level azimuth rectifying means to get tool face angle under constrained conditions for the largest azimuth variation. The constrained conditions have not been considered in several solution methods in the existed literatures, this makes the related calculation formula lack strict mathematic foundation. A standard solving method with constrained minimization problems, Lagrange multiplier method, was used for the new mathematical expression of highest level azimuth rectifying and the calculation formula with optimal solution was derived.