For solution processability of polymer solar cells (PSCs), polymer electron donors are almost always composed of conjugated main chains and flexible alkyl side chains. In this paper, we report a polymer electron donor based on isoindigo units bearing branched oligo(ethylene glycol) (OEG) side chains, P-OEG. Compared with the control polymer bearing alkyl side chains (P-Alkyl), P-OEG exhibits not only a smaller π–π stacking distance and redshifted absorption spectra, but also larger surface energy for better compatibility with the [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) electron acceptor. The PSC device of P-OEG exhibits an open-circuit voltage (V_OC) of 0.73 V, short-circuit current density (J_SC) of 13.92 mA cm⁻², and fill factor (FF) of 0.50, corresponding to the power conversion efficiency (PCE) of 5.10%. This performance is higher than that of P-Alkyl (PCE = 3.0%), which is attributed to the finer phase separation morphology of the P-OEG:PC₇₁BM blend than that of the P-Alkyl:PC₇₁BM blend. These results suggest that the branched OEG side chain is an effective approach to improve the PSC device performance of some polymer electron donors.