In this study, we present a structural optimization framework to design constant force mechanisms (CFMs) with high energy storage capacity. In the framework, the constant force behavior with a zero preload is defined to be ideal, as this has the maximum energy storage given force and displacement limits. A graph-based topology selection, followed by shape optimization is conducted to select designs with energy storage most similar to the energy of the ideal constant force relation. The obtained CFM designs through this framework has a higher energy similarity index compared to typical designs from literature (0.95 versus 0.90). The constant force mechanisms developed through this study can be further applied in different robot/human–environment interfaces that benefit from both mitigating impact force and increasing energy storage.