Thermal batteries are widely employed in various tactical weapons due to their high energy density， wide temperature range， strong instantaneous pulse resistance， short activation time， and long storage life. The output end of the thermal battery is the electrical interface for docking with the missile， and its backflow current during activation directly affects the reliability of the electronic equipment on the missile. Meanwhile， the influencing factors are studied to grasp the variation law of backflow current during thermal battery activation. The research shows that the backflow current is related to the number of parallel battery stack connections， the burning rate of the pilot strip， the burning rate of the heating plate， the stage plate area， the melting rate of the electrolyte， the number of single cells in series of the battery stack， the activation mode of the ignition head， line resistance， and the load power on the missile. This conclusion provides favorable support for designing a safe and reliable power supply circuit for the missile electrical system.