In order to understand the effect of initial free cavity on the violence of confined octogen （HMX）-based PBX-3 in slow cook-off， the weak restricted test setup was designed with initial free cavity volume ratios of 1.0% and 7.4%， respectively， referring to Sandia instrumented thermal ignition（SITI）. The confined HMX-based PBX-3 explosives were heated at the same rate in slow cook-off tests. The temperatures of different locations at the center plane of explosive and shell surface were acquired using small-sized K type thermocouples， the shell velocity after thermal explosion was measured using the heat-resistant probe of photonic-Doppler-velocimeter（PDV）， and the wreckage of test setup was recycled in the slow cook-off chamber. The results show that the confined PBX-3 explosives are ignited in the center region for the same restricted strength and heating process， no matter with the initial free cavity volume. When the initial free cavity volume ratio is 1.0%， the shell surface temperature and the overall temperature of the explosive are higher when thermal explosion occurs， the shell accelerates faster and the maximum velocity is higher after thermal explosion， the wreckage fragments of test setup are smaller， and the reaction violence is higher. It is analyzed when the initial free cavity volume ratio is 1.0%， the stress of explosive is larger before thermal explosion， resulting in more serious damage. When thermal ignition occurs at the center of the explosive， the larger thermal stress causes a higher pressure induced by the gas accumulation， and the burning rate of explosive is higher. The high-temperature gas generated by the combustion is easier to enter the microcracks， resulting in stronger convective burning， faster pressure growth rate and more violent reaction.