To study the formation conditions of protein and polysaccharide composite particles, especially the effect of processing conditions on the construction of their core-shell structure, whey protein isolate (WPI) and chitosan (CS), the only naturally positively charged polysaccharide, were used to form composite particles with protein as core and polysaccharide as shell. The conditions of particle formation and the influencing factors were investigated, and the particle size, potential and microstructure of the formed particles were characterized. The pH value and salt ionic strength were important factors for composite reactions of the two substance. The results showed that when the salt ionic strength was 0 and the pH value reached 5.47, the turbidity of the system reached maxium, which was 98.34% and complexation of the two substance reached the highest degree. Therefore, composite particles prepared at these conditions were used to study the effect of mixing mode, treatment temperature and shear treatment on the composition and particle size of composite particles. The results showed that there was no significant difference in the composition of the composite particles formed by heat-mixing (heat treatment of protein followed by mixing with chitosan) and heat-mixing (mixing of protein and chitosan followed by heat treatment), but the particle size of the particles formed by heat-mixing was smaller and the absolute value of ξ potential was larger, indicating that the particles were more stable. There was no significant difference in the proportion of protein and polysaccharide in the composite particles under heat treatment conditions of 75,85, 95℃, but the higher temperature, the larger particle size and the smaller absolute value of the potential. The particle size could be effectively reduced by shear treatment. The confocal laser scanning microscopy showed that the composite particles formed at 75℃ exhibited a clear co-localization of protein and polysaccharide fluorescence. The results showed that complete core-shell structured composite particles with particle size approximately 744.5nm could be prepared by heat treatment at 75℃, heat-mixing, and assisted with high-speed shearing.