Poly(butyl acrylate) (PBA) side chain equipped isoindigo-bithiophene (II2T) conjugated polymers have been designed and synthesized for stretchable electronic applications. The PBA segment possesses low glass transition temperature and high softness, offering a great opportunity to improve the mechanical property of semiconducting polymer thin films that typically contain lots of rigid conjugated rings. Polymers with 0, 5, 10, 20 and 100% of PBA side chains, named PII2T, PII2T-PBA5, PII2T-PBA10, PII2T-PBA20, and PII2T-PBA100, were explored, and their polymer properties, surface morphology, electrical characteristics, and strain-dependent performance were investigated systematically. The series polymers showed a charge carrier mobility of 0.060.8 cm(2) V-1 s(-1) with an on/off current ratio over 105 dependent on different amounts of PBA chains as probed using a top-contact transistor device. Moreover, we can still achieve a mobility higher than 0.2 cm(2) V-1 s(-1) even if 10% of PBA side chains were added (i.e., PII2T-PBA10). Such PII2T-PBA polymers, more attractive, exhibited superior thin film ductility with a low tensile modulus down to 0.12 GPa (PII2T-PBA20) due to the soft PBA side chain. The more PBA segment was incroporated, the lower modulus was reached. The mobility performance, at the same time, remained approximately 0.08 cm(2) V-1 s(-1) based on PII2T-PBA10 films even under a 60% strain and could be simultaneously operated over 400 stretching/releasing cycles without significant electrical degradations. The above results suggest that the rational design of soft PBA side chains provides a great potential for next-generation soft and wearable electronic applications.