Structure–property relationships of silk is an intriguing topic for silk‐based biomaterials research since these features are related to biomimicking the processing in natural silk fiber formation which results in excellent mechanical properties. Strain‐stiffening is common for spider silks and nonmulberry silkworm silks. However, the structural origin of strain‐stiffening remains unclear. In this paper, the strain‐dependent structural change of Antheraea pernyi silkworm silk is studied by X‐ray fiber diffraction and Fourier transform infrared spectroscopy under stretching. Based on a combination of mechanical and structural analysis, the molecular origins of strain‐stiffening in A. pernyi silk were determined. The relatively high content of the β‐sheets within the amorphous domains in A. pernyi silk is responsible for strain‐stiffening, where “molecular spindles” enhance the extensibility and toughness of the fiber.