In-situ microfibrillation can largely improve the mechanical properties of composites, such as tensile strength and tensile modulus. This approach, therefore, has been well established to manipulate morphology, improve performance of composites and recycle used thermoplastics. However, It is still a challenge to achieve in-situ microfibers in biodegradable poly(butylene adipate-co-butylene terephthalate)(PBAT)/thermoplastic starch(TPS) by injection molding due to the insufficient formation of in-situ microfiber under the weak flow field. In the present work, a novel injection molding technology, named oscillation shear injection molding, was applied to provide an intense shear flow on PBAT/TPS melt in cavity during the packaging stage, in order to impose strong stress on the composite melts to form TPS microfibers. Numerous microfibers of TPS have been successfully achieved in injection-molded PBAT composite for the first time, which allows a good chance to investigate the microfibrillated morphology of PBAT/TPS composite as well as its relationships with mechanical properties. The morphological features of TPS in-situ microfiber in injection-molded parts is clearly observed by scanning electronic microscopy (SEM), which reveals a diameter of the TPS in-situ microfiber around 1 µm. It is amazing that the tensile strength is increased to a nearly double level due to the presence of TPS in-situ microfiber, compared to the conventional injection-molded counterparts, even when the content of TPS is as high as 55 wt%. It is believed that further exploration will give a rich fundamental understanding on the formation of TPS in-situ microfiber in the shear field, aiming to achieve superior biodegradable PBAT/TPS products through the versatile processing such as injection molding process.