Self-assembly nucleators have been increasingly used to manipulate the crystallization of PLLA due to their strong intermolecular interaction with PLLA, while the molecular mechanism of such interaction is still unrevealed. In present work, one special self-assembly nucleator (TMC-300) with relatively high solubility in PLLA matrix, is chosen to investigate how the interaction works at molecular level to promote the crystallization of PLLA mainly through time-resolved spectroscopy. The results indicate that due to the dipole-dipole NH···O═C interaction between dissolved TMC-300 and PLLA, PLLA chains are transformed into gt conformer before TMC-300 phase-separating from PLLA melt, resulting in low energy barrier to pass for the following formation of PLLA α-crystal (α-crystal is consisted of gt conformer). Once the dissolved TMC-300 starts to self-assemble into frameworks upon cooling, the transformed PLLA chains with high population of gt conformer form the primary nuclei on the surface of such self-assembling TMC-300 frameworks. For the first time, not only the heterogeneous nucleation but also the conformational regulation of PLLA chains are proved to be responsible for the high efficiency of the self-assembly nucleators (TMC-300) in promoting the crystallization of PLLA. Therefore, conformational regulation is proposed for crystalline manipulation of PLLA, and this work brings new insight on promoting the crystallization of PLLA even other polymers by regulating their molecular conformation.