Slow cook-off test is one of the key tests of low vulnerability assessment for solid rocket motor. In order to study the influence of the charge size of HTPB composite propellant on the slow cook-off characteristics， slow cook-off tests and numerical simulation were carried out to compare and analyze the ignition growth laws of solid rocket motor under slow cook-off tests， with charge dimensions of Φ100 mm × 200 mm， Φ160 mm × 400 mm and Φ522 mm × 887 mm. Their corresponding ignition temperatures， ignition positions and response levels were determined. Results show that the ignition temperature of specimens of Ф100 mm×200 mm， Ф160 mm×400 mm and Ф522 mm×887 mm of solid rocket motors are 244 ℃， 172 ℃ and 155 ℃， respectively. Taking test data as inputs， the calculated ignition temperature is 250， 269， 154℃， and their corresponding calculation errors and response levelsare 2.88%， 1.17%， 0.64%， and explosion， explosion and deflagration. The calculated cloud diagram shows that the ignition position of medium and small test pieces is located in the center of charge cylinders， and the ignition position of full-scale solid motor is in the center of meat thickness of solid propellant front-end， which is a ring shape area.