In quantum theory, physical amplitudes are usually presented in the form of Feynman perturbation series in powers of coupling constant α . However, it is known that these amplitudes are not regular functions at α = 0 .For QCD, we propose new sets of expansion parameters w k (α s ) that reflect singularity at α s = 0 and should be used instead of powers α k s . Their explicit form is motivated by the so called Analytic Perturbation Theory. These parameters reveal saturation in a strong coupling case at the level α ef f s (α s ≫ 1) = w 1 (α s ≫ 1) ∼ 0.5 . They can be used for quanitative analysis of divers low-energy amplitudes.We argue that this new picture with non-power sets of perturbation expansion parameters, as well as the saturation feature, is of a rather general nature.1 A preliminary version with the main results was published in Ref. [1].