High efficiency and mechanical robustness are both crucial for the practical applications of all‐polymer solar cells (all‐PSCs) in stretchable and wearable electronics. In this regard, a series of new polymer acceptors (PAs) is reported by incorporating a flexible conjugation‐break spacer (FCBS) to achieve highly efficient and mechanically robust all‐PSCs. Incorporation of FCBS affords the effective modulation of the crystallinity and pre‐aggregation of the PAs, and achieves the optimal blend morphology with polymer donor (PD), increasing both the photovoltaic and mechanical properties of all‐PSCs. In particular, an all‐PSC based on PYTS‐0.3 PA incorporated with 30% FCBS and PBDB‐T PD demonstrates a high power conversion efficiency (PCE) of 14.68% and excellent mechanical stretchability with a crack onset strain (COS) of 21.64% and toughness of 3.86 MJ m‐3, which is significantly superior to those of devices with the PA without the FCBS (PYTS‐0.0, PCE = 13.01%, and toughness = 2.70 MJ m‐3). To date, this COS is the highest value reported for PSCs with PCEs of over 8% without any insulating additives. These results reveal that the introduction of FCBS into the conjugated backbone is a highly feasible strategy to simultaneously improve the PCE and stretchability of PSCs. New polymer acceptors (PAs) are developed by embedding flexible conjugation‐break spacer (FCBS) units into the rigid backbones. The incorporation of FCBS affords effective modulation of the crystallinity and pre‐aggregation of the PA and attains optimal blend morphology. As a result, the all‐polymer solar cells exhibit both a high efficiency of 14.68% and excellent mechanical robustness with a crack onset strain of 21.64%.