0D Bismuth nanoparticles (Bi NPs) are grown on the surface of 2D few-layer Nb2CTx MXene to form the mixed-dimensional Nb2CTx@Bi Schottky heterojunction. The large surface to volume area characteristic of 2D Nb2CTx nanosheets can provide the sufficient sites for the growth of Bi NPs, which can not only prevent the aggregation of Bi NPs, but also effectively shorten the length and transmission time of charge transfer. The Schottky heterojunction generated at the interface could lead to the uniflow spreading process of electrons/holes, promotes the separation of photo-induced electron/hole pairs, and suppress the carrier recombination dynamics, thus leading to the enhanced performance of Nb2CTx@Bi-based optoelectronic devices. Systematic investigations demonstrate that Nb2CTx@Bi-based flexible photoelectrochemical (PEC)-type photodetector shows self-powered photodetection capability with fast response/recovery time (0.08/0.08 s), a photocurrent of 1.56 μA cm−2, a responsivity of 585.25 μA W−1, and a specific detectivity of 4.63 × 1012 Jones. The photodetector exhibits self-powered capability after bending 180° for 1000 cycles. Additionally, the repeatable and reproducible ON/OFF signals can be detected after placing the photodetector in alkaline electrolyte for one month, suggesting the potential of Nb2CTx@Bi Schottky heterojunctions for practical applications in flexible optoelectronics.