<p>The deformation mechanism of glycerol plasticized poly(vinyl alcohol) (PVA) with different hydrolyses (88%, 92%, 98%) at elevated temperatures (60–100 °C) was elucidated by <i>in situ</i> synchrotron radiation X-ray scattering. The vinyl acetate (VAc) in PVA acts as a non-crystalline chain defect, which significantly influences the plastic deformation and stretching-induced crystallization behavior of PVA. The key microstructural parameters of PVA during deformation, such as crystallinity (<i>χ</i>_c), lateral crystallite size (<i>L</i>), and long period (<i>l</i>), in combination with the stress-strain curves, were obtained. The experimental results show that the deformation process of the plasticized PVA film present a three-stage evolution: (i) a plastic deformation zone. The plastic deformation of the crystallite occurs as evidenced by the apparent decrease in crystallinity and lamellar reorientation induced by stretching; (ii) the stress softening zone. The decreasing trend of crystallinity becomes slow, and the long period becomes smaller, which indicates that PVA crystallization is induced by stretching; and (iii) the strain-hardening zone. There is a synergistic effect between the crystallite destruction and formation. Further research reveals that a high temperature and low degree of alcoholysis favor the stretching-induced crystallization of PVA, while the system with a high degree of alcoholysis shows significant characteristics of preferred crystal growth.</p>