Manipulating the crystalline morphology of polylactic acid (PLA) is of immense concern in its endpoint application. In this contribution, evolution of crystalline morphology of polylactic acid (PLA) was investigated by combination of home-made fiber-pulling device with polarized optical microscopy (POM). By dint of this, the correlation between shear flow, always existed in the practical processing, and crystalline morphology was expected to be established. The shear rate calculated by Monasse’s method was controlled by changing the pulling speed of the glass fiber (GF) in the PLA melt. In-situ POM results showed that the PLA spherulites were dispersed in the quiescent melt randomly, indicating no nucleation ability of GF. When the shear flow was imposed, that was GF was pulled, a large amount of cylindrulite appeared at the surface of GF. It was because high density of nucleation sites was induced by the shear flow around the GF surface. And these nuclei hindered the lateral extension and forced lamellae growth perpendicular to the GF axis. With the increase of shear rate, the nuclei density on the surface of GF was further increased, resulting in the formation of compact cylindrulite. Fiber pull out test confirmed that the interfacial strength was dramatically improved due to the interface crystallization between the PLA and GF.