Distortional buckling is an important buckling mode for cold-formed lipped channel sections. The distortional buckling behavior depends mostly on the rotational restraint at the web/flange juncture. This paper focuses on cold-formed lipped channel sections under strong axised- moment. Based on a displacement function which fits all boundary conditions, an approximate analytical solution of the rotational restraint provided by web is firstly provided by Galerkin method, whose accuracy is then confirmed by finite element method. Finally, an accurate simplified expression is proposed. The results of the present studies are expected to be useful to the future development of a simple approximate formula for the distortional buckling load of cold-formed lipped channel columns subjected to moment around strong axis.