Currently, porous polymeric membranes are mainly produced by the NIPS and TIPS techniques, but both have intrinsic technical limitations in terms of effective control of membrane structures. Recently, a novel Combined solvent Crystallisation and polymer Diffusion (CCD) method has been established to produce high-performance membranes with a unique asymmetric structure, where solvent nucleation and crystallisation in a binary polymer-solvent system are utilised to serve as the pore-forming mechanism. However, the membrane formation mechanism of the CCD method has yet been understood fully. In this work, the formation mechanism is proposed based on the widely acknowledged principles of nucleation and crystal growth. A typical and commonly used amorphous polymer, polyethersulfone (PES) is employed as a sample membrane material to prepare microfiltration/ultrafiltration membranes using the CCD method and the effect of cooling rate on the membrane structure is investigated. The structural features of the membranes can be well explained using the proposed membrane formation mechanism, where the effect of cooling rate is rationalised. Pristine PES membranes with pore sizes < 20 nm and narrow pore size distribution can be achieved when a fast cooling rate is applied. Such membranes show a high pure water permeation flux, which is comparable to the nominal flux of commercial hydrophilic PES membranes with similar pore size.