Here we present a unified theory of above threshold ionization (ATI) phenomena. Based on this theory, we can describe ATI processes in realistic atomic screen potentials and can also elucidate experimental observations of ATI phenomena. The ATI phenomena mainly involve two successive procosses: (1) Quantum transition processe from an initial field-free state to final strong laser field dressed states. (2) in order to descibe experimental observations, one must consider post-interactions between the escaping photoelectron and the strong laser fields, which are classic processes owing to ponderomotive forces generated by non-uniform laser fields.