Abstract: Multiaxial failure analyses of highly filled particle-reinforced viscoelastic materials currently rely largely on instantaneous strength theories such as the Mises criterion, which generally neglect the effects of strain rate, large deformation, and creep. Consequently, these approaches are inadequate for describing the coupled effects of strain rate, stress state, and loading history on damage evolution under complex multiaxial stress conditions. In this study, hydroxyl-terminated polybutadiene (HTPB) solid propellant, a typical highly filled particle-reinforced viscoelastic material, was selected as the research object. Biaxial tensile tests were conducted at 25℃ over a wide strain-rate range from 10⁻⁴ s⁻¹ to 1 s⁻¹ under three loading ratios of 1∶1, 1∶0.5, and 1∶0.25. The results show that failure of the material is primarily governed by interfacial debonding, matrix tearing, and particle fracture. With increasing strain rate, the dominant failure mechanism shifts from interface-controlled damage to particle fracture. Based on Miner’s linear cumulative damage concept, a biaxial tensile failure criterion incorporating strain-rate effects and loading history was developed by introducing mechanical parameters including strain rate, stress triaxiality, the second invariant of the deviatoric stress tensor, and maximum principal stress. On the basis of this criterion and the principle of equivalent damage, failure envelopes capable of reflecting the evolution of damage mechanisms at different strain rates were constructed. The model predictions agree well with the experimental results, with a mean absolute error of 13.66% in constant-load validation and an average normalized distance error of 3.49% for the failure envelopes. Moreover, the proposed model exhibits higher overall predictive accuracy than the conventional Mises criterion. This study provides a unified characterization of loading-history-dependent and rate-dependent damage behavior under biaxial stress states, offering a reference for failure prediction of highly filled particle-reinforced viscoelastic composites under complex biaxial loading conditions.