Abstract: In order to clarify the complex phase behavior change mechanism between plant protein and polysaccharide, soy protein isolate aggregates (SPIA)/guar gum (GG) mixed solution was used as a model system, and the phase separation behavior (including macroscopic morphology and microstructure) and rheological properties of the mixed solution at different NaCl (0~75 mmol/L) and GG (0~0.2wt%) concentrations were studied. The results showed that the increase of NaCl and GG concentration significantly (P<0.05) increased the turbidity and viscosity of SPIA/GG mixed solution, and the degree and speed of phase separation of the samples were increased by macroscopic observation and time scanning. The SPIA/0.1% GG mixed solution showed a protein external phase, and the SPIA/0.2% GG mixed solution showed a bicontinuous phase morphology. The addition of GG resulted in a decrease in the content of β-sheet in the secondary structure of SPIA. With the increase of NaCl concentration, the content of β-sheet decreased first and then increased, and the cross-linking between amino acids was promoted, resulting in the change of tertiary structure. This study explained the regulation mechanism of NaCl and polysaccharide concentration on the phase separation of plant protein/polysaccharide mixed system, and provided a theoretical basis for the development of different microscopic forms of protein-polysaccharide mixed system food.