Revealing the molecular chain dynamics and determining the absolute degree of crystallinity during the phase transition have been a challenge in the field of polymer crystallization. In this work, a special low-temperature testing chamber was designed and utilized in combination with a laboratory X-ray diffraction apparatus, in order to perform temperature-dependent time-resolved WAXD measurements of highly flexible poly (dimethylsiloxane) (PDMS) elastomer. From the changes of diffraction patterns and the 2D correlation analysis of WAXD profiles, we can clearly observe the crystallization and melting behaviors of PDMS during cooling and heating. Meanwhile, an improved multi-Gaussian fitting method was developed and applied to quantitatively analysis the structural evolution. Moreover, the constrained amorphous interchain segments show a narrow fitted Gaussian peak like crystalline planes, which can only be discovered when the crystallinity of PDMS exceeds a certain threshold of about 10 %. These results have evidenced that this improved method enables not only an accurate determination of crystallinity, but also a reliable quantification of structural evolution of PDMS elastomeric materials, which might be beneficial for understanding the mechanism of phase transition.