In response to the failure issues of HTPB solid propellant under varying confining pressure and strain rate conditions, a modified form of the twin-shear strength criterion that accounts for the influence of confining pressure was proposed. This modification was based on uniaxial tensile experiments conducted on HTPB propellant at different strain rates and confining pressures, and subsequently compared with the generalized form of the twin-shear strength criterion. Additionally, considering the strain rate dependency of solid propellants, a modified form incorporating both confining pressure and strain rate for the twin-shear strength criterion was suggested. The study findings indicate that this modified form provides a more precise description of solid propellant failure under confining pressure compared to its generalized counterpart. Furthermore, when applied across various strain rate conditions, this modified criterion demonstrate high accuracy in predicting HTPB propellant failure.