Proteins can selfassemble to form nanofiber polymers under unconventional conditions. This kind of polymer structure given protein monomers and their conventional polymerization formed with highlevel functional properties, but this selfassembly process was easily interfered by other proteins and destroyed the assembly and polymerization mode. The interaction of two main protein components in milk was explored, and the effect of casein (CN) on the selfassembly of whey protein concentrate (WPC) to form fibril polymers was studied, and the degree of interference of WPC by CN at different stages of selfassembly was analyzed. The results showed that incorporation of CN at different stages of fibril formation determined the structure and shape of the hybrid copolymer. The earlier CN was added, the more obvious the inhibitory effect on the fibril formation was. With the increase of CN concentration, the mixing time point of CN for the formation of fibril polymer was delated. Among them, the surface hydrophobic interaction and the formation of disulfide bonds promoted the polymerization of CN and WPC, which weakened the interaction between WPC itself, and then inhibited the selfassembly of WPC fibrils. This inhibition was particularly obvious when CN was mixed in the initial stage of fibril formation. The polymerization rate constant k and the polymerization ability of WPC and CN were increased and the polymerization between WPC was weakened. The formation of WPC fibril polymer was destroyed by the interference of CN. The results indicated that cows milk protein cannot form nanofibril polymers due to the effect of CN in cows milk, but the casein in cow’s milk was removed, and mixed in the middle and late stages of WPC selfassembly polymerization did not affect the formation of nanofibril polymer structure. 