In this talk, we give a multifractal analysis on the Birkhoff average for a class of skew product transformations, which is driven by a unique ergodic homeomorphism and satisfies Anosov and topological mixing on fibers property. The conclusion is twofold: a variational principle between the fiber Bowen’s topological entropy on conditional level sets of Birkhoff average and fiber measure-theoretical entropy; If the irregular set is nonempty on some fibers, then on almost every fiber with respect to the unique ergodic measure, it is nonempty and carries full fiber Bowen’s topological entropy. Examples of systems under consideration are provided.