Poly(thienylene vinylene)s (PTVs) are close analogs to the extensively studied polythiophenes and the insertion of one double bond between every two thiophene units along the main-chain of PTV backbone eliminates steric repulsion and potentially allows installment of conjugated side-chains without significantly distorting the main-chain planarity, offering a facile way of fine-tuning the polymer electronic properties through main-chain/sidechain cross-conjugation. We describe here a new methodology that combines acyclic diene metathesis and post-polymerization modification methods for the access of a series of cross-conjugated PTVs bearing aromatic side-chains connected to the main-chains through alkyne spacers. Compared with unfunctionalized PTVs, these cross-conjugated PTVs showed broader absorption ranges, reduced bandgaps, and rarely observed fluorescence. Organic solar cells employing the cross-conjugated PTVs displayed up to double the efficiency improvement over those employing pristine PTVs. For the first time, these PTVs were applied as sensitizers in composites with g-C3N4 for photocatalytic hydrogen production. It was found that the relative energy level positions between the polymers and g-C3N4 and polymer crystallinity likely played decisive roles in determining the hydrogen production efficiencies, which gave us useful guidelines on future optimization of polymer structures and selection of photocatalytic systems.