During the preparation of ultra-high-molecular-weight polyethylene(UHMWPE) microporous membranes, a large number of low-molecular-weight diluents such as liquid paraffin (LP) were added to reduce the viscosity of polyethylene melts, and it was used as pore-forming agent for polyethylene microporous membranes. This paper firstly proved that UHMWPE/LP blend belongs to the strong interaction system through the analysis of thermodynamics theory.Then the phase equilibrium temperature curve was obtained in slow heating and slow cooling process through high precision hot stage microscope device, while the melting curve and crystallization temperature curve were obtained by differential scanning calorimetry (DSC). It is found that the phase equilibrium temperature curve and crystallization temperature curve are proportional to the content of polyethylene, which are regard as the curves of the beginning temperature and the end temperature of phase transition in the thermodynamic phase diagram of UHMWPE/LP blends, respectively. The phase separation process occurs in the temperature range of 100～130 ℃. The solid-liquid(S-L) phase separation of UHMWPE/LP blends is resulted from the crystallization of polyethylene, and the phase separation process of UHMWPE/LP blends is considered as the process of material exchange between crystalline phase and amorphous phase. Finally，through the SEM pictures of microporous membranes prepared by adding different content of paraffin oil，it is found that liquid paraffin in UHMWPE/LP blends could weaken the crystallization ability of polyethylene and affect the size of the amorphous phase, which could change the micro-structure of polyethylene microporous membranes.