<p>The force–displacement response and specific energy absorption are important indicators for evaluating a structure’s ability to protect occupants during crash events. The development of methods to pre-customize the force–displacement response can provide new design ideas for crashworthiness in aircraft structures. This paper proposed a crashworthiness optimization method based on variable wall thickness. Firstly, the analytical relationship between the external crash force and element thickness is established using work-energy theorems and classical plastic hinge theory to achieve the customized force–displacement response. Secondly, specific energy absorption is incorporated into the optimization objective to improve material utilization and explore the potential for structural light-weighting. Finally, the divide regions method, which is based on thickness thresholds, is combined to overcome the manufacturing difficulties associated with free-size elements. This method is numerically validated using a C-channels strut under axial loading and a C-channels fuselage frame under lateral loading. The results show that the optimized structure exhibits more stable force–displacement response than the initial design, and material utilization is improved while ensuring manufacturability. The major crashworthiness indicators listed in the paper have been improved. The improvement in crush load efficiency (increased by more than 45%) for measuring load fluctuations is the most significant, while the specific energy absorption has also increased by more than 11%. The results demonstrated the method’s great advantage in the crashworthiness design of thin-walled aircraft structures.</p>