Accelerated aging tests under pre-strain were conducted on HTPB-based composite solid propellant with the goal of investigating the effect of pre-strain aging on its damage properties. A statistical damage constitutive model based on continuum damage theory and statistical strength theory was established. The aging damage coefficient, making aging process of propellant equivalent to a form of damage, was introduced to correct the damage variable. Experimental results show that theoretical model has good agreement with experimental results and can accurately describe the mechanical behavior of propellant during prestrain aging. Further analysis indicated that the damage effects caused by pre-strain can be identified from the equation of the aging damage coefficient. Aging time influences both tensile strength and shape characteristics of the stress-strain curve of propellant in the damage stage, while pre-strain only decreased the tensile strength. The strain damage threshold value decreased linearly over the aging period and with increasing pre-strain level during the aging process.