In order to study the interaction between Tremella fuciformis polysaccharide (TFP) and soybean protein isolate (SPI), the complex coacervation process between them was characterized by turbidity measurement as well as using a nano particle size analyzer, a laser confocal scanning microscope, a rheometer and Fourier transform infrared (FTIR) spectroscopy. The results showed that pH, the ratio of SPI to TFP and salt concentration had great influence on the formation of coacervates. When the SPI/TFP ratio was larger than 1, the critical pH associated with complex formation (pH_c and pH_φ1) shifted to higher pH. When the SPI/TFP ratio was less than 1, pH_φ1 shifted to a lower level. The maximum absorption peak for turbidity gradually decreased (pH_opt gradually decreased) with increasing level of TFP. When the SPI/TFP ratio was 1, the interaction between them was the strongest at pH 3.0, and the viscosity of the system was the largest (1.30 Pa·s). In addition, the largest amount of coacervates was formed with the largest particle size. The formation of SPI-TFP coacervates was promoted by low salt concentrations (c(NaCl) < 20 mmol/L) but inhibited by high salt concentrations (c(NaCl) ≥ 20 mmol/L) due to shielding. When the shear frequency was 10 Hz, the NaCl concentration was 20 mmol/L, and the SPI/TFP ratio of 1:1, the highest viscoelastic modulus was obtained, along with storage modulus of 9.24 Pa and loss modulus of 3.40 Pa. This study may provide a theoretical basis for the application of TFP in plant protein based beverage.