New siloxane-imide block copolymers (PSI) were designed and synthesized from 2,2′-bis[4-(4-amino-phenoxy) phenyl] hexafluoropropane (BDAF), 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and diamine-terminated poly(dimethyl siloxane)s (SIDA, average molecular weight = 900, 1680 and 4600) for application in organophilic pervaporation. The chemical structure of the copolymer was confirmed by FT-IR and 1H NMR spectra. In addition, its good thermal stability and micro-phase separated amorphous structure were confirmed by TGA, DSC, WXRD and AFM analyses. The content of siloxane blocks in the copolymer decreased, and thus the micro-phase separation became more serious with decreasing the molecular weight of SIDA used for copolymerization. The PSI membrane prepared from the lowest molecular weight of SIDA (900) was fragile. The siloxane blocks in the copolymer mainly contributed to the pervaporation properties of the copolymer membranes. Their increase made an improvement of the separation properties including permeate flux and selectivity. But simultaneously, it would lead to a drop of mechanical strength of the corresponding copolymer membrane.