Recently, conjugated donor–acceptor block copolymers (CDABPs) were synthesized by one-pot polymerization and applied in polymer solar cells with a single-component active layer. These polymers can suppress phase separation and exhibit inert internal morphologies compared to the polymer blends; however, their thermal stability remains an issue to be addressed. In this study, we successfully synthesized a new crosslinkable CDABP (P(OXBTT-co-NDI2T)), comprising a donor block bearing photocrosslinkable oxetane side chains and an acceptor block to improve the thermal and morphological properties. The crosslinked P(OXBTT-co-NDI2T) film showed high solvent resistance, and it was confirmed that even the P(NDI2T) block without a crosslinking unit did not dissolve after washing with a solvent. This indicates that the P(NDI2T) block was obviously covalently bonded to the P(OXBTT) donor block in the CDABP structure. In particular, compared to the blend (P(OXBTT) and P(NDI2T)) film, the P(OXBTT-co-NDI2T) film exhibited a uniform surface morphology. Consequently, the polymer solar cell based on the crosslinked P(OXBTT-co-NDI2T) active layer showed higher power conversion efficiency and better thermal stability than that based on the blend film.