Self-forming dynamic membrane was used to separate high-concentration sludge fermentation liquid, and the formation mechanism and separation efficiency of dynamic membrane were investigated. The results indicated that the impact of sludge concentrations was negligible on the formation of dynamic membrane. Though membrane flux could be influenced by sludge concentration at the initial stage of membrane formation process, the influence was not obvious at the stable stage. Membrane flux was improved with increasing filter cloth pore size and stirring speeds. Moreover, the results indicated that the formation process of dynamic membrane followed the dead-end filtration model, which could be divided into four stages. Firstly, filter cloth pore was blocked by those sludge particles with the diameter similar to the pore size of filter cloth. Secondly, sludge particles formed monolayer sludge on the filter cloth. Thirdly, sludge particles formed multilayer sludge on the filter cloth. Finally, large sludge particles deposited onto the sludge layer. After formation of the dynamic membrane, the retention efficiency of sludge particles and SCOD could reach 98% and 28% , respectively, and the permeation efficiency of VFAs was over 82%. Proteins in EPS were the main component of the dynamic membrane.