The microporous structure of ultra-high-molecular-weight polyethylene (UHMWPE) separator is originated from the solid–, Liquid (S–, L) phase separation in the UHMWPE/Liquid Paraffin (LP) blend. To understand the pore-forming mechanism, in this study, we have explored the dynamics of S–, L phase separation in the UHMWPE/LP blends with different compositions by DSC analysis. The phase separation temperature region of 104–, 120 °, C is divided into six temperature points for obtaining isothermal crystallization curves. Then, the data are analyzed using Avrami equation and Arrhenius equation, as a result, the activation energy at high temperature region is reached 732. 3–, 569. 4 kJ mol−, 1, higher than 118. 1–, 149. 9 kJ mol−, 1 at low temperature range. It means that initial phase separation could be more difficult for less effective nucleation at high temperature. In addition, the value of "n" also drops down with increasing the UHMWPE content in the blends, which is consistent with two-dimensional silk-like and one-dimensional needle-like morphology in the case of 30% UHMWPE content. Low content of UHMWPE segments has a weaker nucleating ability, which is favorable for the crystal growth in the UHMWPE/LP blends and generating loose porous structure. It is also inspiring for the production of separator to regulate the pore structure in the phase separation.