Abstract: A design method based on equilong section lines for tool surface in complex aircraft skin stretching was proposed to improve the strain distribution of blank during stretching, against the lack of practical and detailed design methods for tool surface in complex aircraft skin stretching. In this method, section lines of tool surface along stretching direction were designed as equilong as possible by adjustment of process addendum to gain approximate elongations on all section lines of blank after stretching, and then curvature and tangent directions were adjusted to obtain uniform strain distribution on blank after stretching and decrease or avoid appearance of failings relative strain distribution. This design method was verified to be of feasibility and validity by the design of a tool surface for a complex aircraft skin stretching and the strain distribution obtained by finite element method (FEM).