Comprehensive Summary The development of conjugated polymer photocatalysts for efficient solar‐to‐hydrogen energy conversion is highly desirable for the sustainability of our society. Although the construction of donor‐acceptor (D‐A) structure in conjugated polymer photocatalysts for solar‐to‐hydrogen energy conversion has been well documented, less attention has been paid on how large D and how large A units combined together could achieve the best performance. Herein, a series of D‐A copolymers P(BDT‐DBTSOx) (x = 7, 19, 39, and 79) composed of a benzodithiophene (BDT) donor unit and an oligomeric dibenzob,dthiophene sulfone (DBTSO) acceptor segment were synthesized and studied. It was found that the polymer photocatalytic stabilities under full‐arc irradiation improved upon shortening the length of the acceptor segment. Under visible light irradiation and in the presence of 3 wt% Pt cocatalyst, P(BDT‐DBTSO79) displayed the best performance with an optimal hydrogen evolution rate of 119.3 ± 5.8 mmol·g–1·h–1. This is 1.4‐fold as that of DBTSO homopolymer and 22.5‐fold as that of BDT/DBTSO alternative copolymer, highlighting the importance of acceptor length in D‐A structure for achieving high photocatalytic performance. Photocatalytic performance of P(BDT‐DBTSOx) polymer in visible light‐driven hydrogen production from water can be finely tuned by adjusting the length of acceptor segment in its structure. And a remarkable hydrogen evolution rate as large as 119.3 ± 5.8 mmol·g–1·h–1 has been successfully achieved.