Polylactide (PLA) exhibits various types of crystal modifications depending on the preparation conditions, including the components. To solve the open question, a reliable calculation method for crystallinity, crystal forms, and composition in neat PLA and PLA composites was developed on the basis of temperature-dependent synchrotron wide-angle X-ray diffraction results. The relative composition of amorphous, α-form, and α’-form phases of PLA and its composites filled with halloysite nanotubes during heating was successfully obtained. It was found that only 47–56% of α’-form crystals transform into α-form crystals during a 2 °C/min heating process for PLA with a molecular weight of 54,300 g/mol. The loading of halloysite nanotubes decreases the cold crystallization and starting transition (α’ crystals transform into α-form crystals) temperatures of PLA. The crystallinity and the main diffraction peak intensity as a function of temperature were also analyzed. These results suggest that the α’-to-α form transition is a solid-solid phase transition.