Conjugated polymers featuring tunable band gaps/positions and tailored active centers, are attractive photoelectrode materials for water splitting. However, their exploration falls far behind their inorganic counterparts. Herein, we demonstrate a molecular engineering strategy for the tailoring aromatic units of conjugated acetylenic polymers from benzene- to thiophene-based. The polarized thiophene-based monomers of conjugated acetylenic polymers can largely extend the light absorption and promote charge separation/transport. The C≡C bonds are activated for catalyzing water reduction. Using on-surface Glaser polycondensation, as-fabricated poly(2,5-diethynylthieno[3,2-b]thiophene) on commercial Cu foam exhibits a record H₂-evolution photocurrent density of 370 μA cm⁻² at 0.3 V vs. reversible hydrogen electrode among current cocatalyst-free organic photocathodes (1–100 μA cm⁻²). This approach to modulate the optical, charge transfer, and catalytic properties of conjugated polymers paves a critical way toward high-activity organic photoelectrodes.