The ability to render the cocrystallization over microphase separation in all-conjugated block copolymers represents an important endeavor towards achieving enhanced charge transport. This, however, remains a grand challenge, particularly in all-conjugated triblock copolymers. Herein, we report the unravelling of the dependence of cocrystallization in all-conjugated triblock copolymers on a set of internal structural parameters, and more importantly, the scrutiny of the correlation of their unique cocrystalline structures to charge transport properties for organic field-effect transistors (OFETs). Specifically, a series of poly(3-butylthiophene)-block-poly(3-alkylthiophene)-block-poly(3-hexylselenophene) triblock copolymers (denoted P3BT-b-P3AT-b-P3HS) are meticulously designed and synthesized. Intriguingly, a shorter alkyl side chain length and a shorter main chain length of the central P3AT, as well as a stronger cocrystallization ability of the two outer blocks (P3BT and P3HS), are found to favor the cocrystallization of the three dissimilar blocks in P3BT-b-P3AT-b-P3HS. Notably, the charge transport properties of P3BT-b-P3AT-b-P3HS correlate strongly to their various crystalline structures, thereby imparting their utility for high-performance OFETs. This study highlights the robustness of meticulous molecular engineering of all-conjugated multiblock copolymers in tailoring their cocrystallization behavior and in turn charge transport characteristics that underpins their advances in optoelectronic materials and devices.