The molecular interaction mechanism between Tremella polysaccharide (TP) and whey protein (WP) was investigated, and a novel composite beverage was developed based on this mechanism. TP-WP composite systems with different mass ratios (ranging from 1:4 to 4:1) were constructed, and their characteristics were analyzed using multi-scale techniques. The results showed that as pH decreased from 7.0 to 2.0, the turbidity of the composite systems initially increased and then decreased. The maximum turbidity for WP alone was observed at pH value 5.0 (OD600=0.433), while the addition of TP caused a leftward shift in the phase transition critical point from pH 5.0 to 4.0. Under neutral conditions, stable TP-WP complexes were formed primarily through electrostatic interactions and hydrogen bonding. At a TP:WP ratio of 4:1, the complexes exhibited a Zeta potential of -19.12 mV, which was slightly lower in absolute value than that of the WP-alone system (-19.56 mV). However, the average particle size was significantly reduced by 24.24% to 384.61 nm compared to WP alone (507.69 nm), and the polydispersity index (PDI) decreased from 0.94 to 0.51, indicating enhanced stability of the composite system. Based on this mechanism, the beverage formulation was optimized through orthogonal experiments. The optimized formulation was composed of 60 wt.% Tremella extract, 2 wt.% WP hydrolysate, 5 wt.% soft sugar, 3 wt.% milk powder, and 0.2 wt.% carboxymethyl cellulose. The final product demonstrated a turbidity change rate of <5% and a centrifugal precipitation rate of <1% over 30 days, with a coherent flavor. Its microbial indicators were verified to comply with national standards. These findings provide a theoretical and technical foundation for the high-value utilization of WP and the development of functional Tremella-based products.