Controlling the block ratios, and thus the material properties, of fully conjugated block copolymers (BCPs), remains one of the challenges to implementing BCPs in the field of organic electronic and photonic devices. Here, the synthesis of a fully conjugated BCP, P3HT-b-PTB7, was demonstrated in a customized flow reactor for the first time. The PTB7 block was synthesized by Stille polycondensation using a P3HT-Br macroinitiator and the reaction parameters (reaction time and the amount of injected P3HT-Br) were modified to precisely control the block ratio. Specifically, the P3HT : PTB7 block ratios ranged from 50 : 50 to 23 : 77 (w : w). We also confirmed the high reproducibility of the block copolymerization by obtaining a standard deviation of 2.94% in the number-averaged molecular weight through three repeated runs of the BCP synthesis. These BCPs exhibited distinctive optical and electrochemical properties. Furthermore, we conducted investigations into their photovoltaic properties, revealing a significantly enhanced short-circuit current (J_sc) in organic solar cells when using BCP as the donor material in the active layer, compared to devices employing homopolymers or a physical blend of homopolymers as the donor. These results highlight the promising potential of using flow syntheses to precisely control block ratios of conjugated BCPs in a reproducible manner, thereby advancing high-performing organic semiconductors towards industrial applications.