It is shown that all of the usual programs for the operator expansion can be performed in terms of string operators on the light cone; namely, the separation of contributions from large and small distances, the study of higher twist corrections and the renormalization group analysis. Evolution equations for the leading-twist QCD string operators such as g ̄ y(x)P exp(ig∫ C A μ dx μ)ψ(0) are studied in the coordinate space, which has an advantage of preserving explicitly the Lorentz and (in one-loop) conformal invariance of the theory. The solution is found, relating the two string operators in different normalization points. Its short-distance expansion reproduces conventional results for the summation of leading logs in deep inelastic scattering and evolution of hadron wave functions. The light-cone expansion of string operators provides an effective covariant technique for a separation of higher twist effects. As an illustration we calculate the twist-three and twist-four corrections for the deep inelastic scattering and confirm in this way the recent results on the renormalization of twist-three operators.