A design method for designing a part made from 3D woven composite material, implemented by computer, the design method comprising: a step (F1) of obtaining shape data (SHAPE) representing the outer surface of the part, for each point (PT) of a set of points of the outer surface, a step (F5, F6) of determining a distance (d) between said point and the projection of said point onto a target surface, according to a predetermined projection direction, and a step (F8) of determining the structure of a 3D woven preform depending on the determined distances (d), characterised in that it comprises a step (F2) of obtaining projection data (F) indicating a projection direction depending on the position of a point on the outer surface of the part, wherein, for at least one portion of the points (PT) of the set of points, during the step (F5) of determining a distance, the projection direction used is determined according to said projection data (F) and the position of said point.