The invention relates to a composite thermotropic phase isolated membrane preparation method which is characterized in that used composite diluent is the mixture of water-soluble good solvent and water-soluble additive of polymers. The membrane forming separation is a composite process which mainly takes the thermotropic membrane phase separation as the dominant role and non-solvent induced phaseseparation as the subsidiary role. The membrane prepared through the method has the advantages of high abruption strength, high abruption elongation and high water flux. The method is applicable to the preparation of membranes which are made of a plurality of polymers, including polyvinylidene fluoride, polysulfone, polyether sulfone, polyvinyl chloride and polyacrylonitrile. The performances of the membranes are improved greatly. Thus, the membranes can be widely applied to the fields of drinking water advanced treatment, pretreatment before reverse permeation, membrane bioreactor wastewatertreatment, reclaimed water reuse, etc.