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Stimulated by the requirement for wearable electronics, crack‐based strain sensors made from polymer‐supported metal films have been reported as a prospective structure for detecting subtle deformation with ultrahigh sensitivity and excellent flexibility. However, the regulation of crack preparation remains a challenge and the use of noble metals retards the large‐scale promotion toward practical application. Here, a cost‐effective strain sensor with ultrahigh strain sensitivity under small strains (ε) is designed based on the submicron to nanoscale voided clusters in Cu‐Al alloy films. The formation of channel cracks in the film is manipulated by regulating the intrinsic microstructure of the film, and the corresponding mechanism is discussed in detail. The strain sensor developed from the cracked Cu‐Al film exhibits ultrahigh gauge factors as high as 584 (0% < ε < 0.5%), 10219 (0.5% < ε < 0.9%), 43152 (0.9% < ε < 1.75%) which is among the highest in reported values. Furthermore, the practical application of the developed sensor in wearable electronics and detection of small deformation is demonstrated. This work provides a novel approach to optimize the sensing performance of flexible strain sensors.

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