theory of tailing wavetrain generation in the precursor soliton generation in single-layer flow is pre-sented in terms of Whitham s averaged method in the present paper. The group characters of the tailing wavetrain gener-ation are represented by the evolution equations of roots of a cubic algebraic equation resulting from the fKdV equation inthe single-layer flow without source term. Based on the evolution equations, the group velocity of the tailing wavetrain isfound theoretically, furthermore a theoretical solution of the tailing wavetrain generation is found in terms of the evolutionequations. To examine the theoretical results, a numerical calculation of fKdV equation in single-layer flow is carried outin the laboratory frame, at the same time a comparison is also done. The comparison between theoretical and numericalresults shows they are in good agreement.