Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) is a polyhydroxyalkanoate (PHA) with broad application prospects in various biomedical fields. Our previous study demonstrated the great biocompatibility of PHBHHx with neural stem cells (NSCs) in vitro. In this study, we went a step further and implanted the NSC-carrying PHBHHx film to in vivo into the lesion region in a rat traumatic brain injury (TBI) model. The in vivo biocompatibility of PHBHHx, as well as the survival, migration and differentiation of the transplanted NSCs were investigated. The results showed that PHBHHx did not induce additional reactive gliosis and supported the in vivo survival and growth of the transplanted stem cells. The transplanted NSCs were able to migrate into the brain tissue and differentiated into both neurons and astrocytes. Moreover, PHBHHx seemed to be able to maintain the sternness of the attached NSCs both in vitro and in vivo. The major monomer of PHBHHx, 3-hydroxybutyrate (3-HB), might contribute to this bioactivity because the addition of low concentrations of 3-HB facilitated the self-renewal of NSCs by shortening the G1 phase, promoted their proliferation, and enhanced the expression of a key regulatory transcription factor which helped to keep the stemness of NSCs. These results highlight the application of PHBHHx as a promising bioactive material for NSC transplantation in the brain and open up a wide space for further studies on functional recovery following the transplantation of NSCs attached to PHBHHx.