Molecular dynamics simulation has been employed to study the encapsulation of boron nitride–graphene nanoribbons (BNCNRs) in a single-walled carbon nanotube (SWNT). The simulation results show that a helical BNCNR with large curvature can uncoil repeatedly and spontaneously in the SWNT, like the unwinding of the DNA in the nucleus. The uncoiling of the BNCNRs is accompanied by a system energy exchange between non-bonding energy and elastic potential energy due to the competition between the induction of graphene nanoribbon (GNR) segments and the resistance of boron nitride nanoribbon (BNNR) segments. The electronic transmission capacity of the BNCNR changes with the helical angle of the BNCNR, suggesting a changing electrical signal in the uncoiling and spiraling process. This study provides the opportunity to understand the encapsulation of the BNCNR in the SWNT in detail.