A method to obtain the impact strength of triple-base gun propellant is proposed according to strain energy density. Using a drop-weight impact testing machine, the impact strengths of DAGR125-21/19 gun propellant at room temperature (20 degrees C) and low temperature (-40 degrees C) were studied. In the experiments, there were different impact energies via changing the drop weight or height. The surface images of gun propellants during the impact process were captured using a high-speed camera. The deformation of DAGR125-21/19 gun propellant was calculated through digital image correlation method (DICM). After analyzing the surface images and deformation, it was found that there was a good agreement between the maximum of axial nominal strain energy density (ANSED) and initial fracture point. Finally, the impact strength of DAGR125-21/19 gun propellant was obtained through calculating the strain energy density during the plastic stage between the yield and initial fracture points. Moreover, successive parallel and replication experiment results proved the rationality and feasibility of the proposed method to determine the impact strengths of DAGR125-21/19 gun propellant at room temperature (20 degrees C) and low temperature (-40 degrees C).