Thermal explosion tests of double-base propellant charges and modified double-base propellant charges with diverse diameters were studied to explore the feasibility of evaluating the thermal safety of propellant charge by the critical temperature of thermal explosion (T_cr) and appropriate test conditions and data processing methods. The time to ignition and critical temperature of thermal explosion of propellant charge occurred at certain temperature were obtained. Results show that T_cr can be used to evaluate the thermal safety of propellants and the appropriate test diameter range is from 20 mm to 25 mm. Assumptions that the maximum test time is 10h and the test can be ended as the difference between the lowest explosion (ignition) temperature and the highest non-explosion (no-ignition) temperature is less than 5 ℃ for double-base propellant and modified double-base propellant are reasonable. The values of T_cr predicted by extrapolating through fitting equation of lnr-T_cr are lower than those predicted by other fitting methods cited in references.