Disclosed is a device for separating a propulsion system from a missile, the device including a locking unit configured to fix a propulsion system to a missile, a string disposed to cross a rear portion of the propulsion system to be broken by heat of the propulsion system, and an unlocking unit configured to unlock the missile by the broken string, whereby the number of additional components of an unlocking system can be reduced as many as possible so as to simplify the configuration of the apparatus, resulting in guaranteeing an enhanced performance of the missile, an easy assembly of the missile, and a reduction of a fabricating cost.