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In order to study the effect of charge size on the cook-off characteristics of pressed charges, the calculation model of cook-off process was established for HMX-based pressed charges. The cook-off bombs with different charge sizes were simulated by Fluent software, the effect of charge size on the ignition position, response temperature and response time of pressed charges at different heating rates was calculated. It was found that, at the same heating rate, the response temperature of the charge center is the highest when the length-diameter ratio of HMX-based pressed explosive is 1.0, and the ignition temperature of the charge center decreases with the increase of the length-diameter ratio when the length-diameter ratio is greater than 1.0. When the length-diameter ratio increases to a certain extent, the response temperature of the charge center tends to be a constant. The ignition position of the charge is determined by both the heating rate and the size of the charge, and the ratio of heat transfer between the end face and the periphery face of the charge is inversely proportional to the square of the length-diameter ratio. When the heating rate is slow or the length-diameter ratio is small, the ignition position of the charge is located at the charge center; when the heating rate is fast and the length-diameter ratio is large, the ignition position of the charge is gradually away from the charge center. [ABSTRACT FROM AUTHOR]