A bag for emplacing a combinatorial billiard cue is used to emplace a disassembled combinatorial billiard cue, including a bag unit which contains a billiard cue bottom seat and a billiard cue upper rack. A dent at a bottom of the billiard cue bottom seat to emplace a cue segment includes an extension mechanism contracted vertically when being pressed at a first time and extended vertically when being pressed at a second time. Each cue segment of the combinatorial billiard cue is emplaced from a through-hole of the billiard cue upper rack, putting a bottom of the cue segment in a dent of the billiard cue bottom seat and above the extension mechanism. After the cue segment is pushed down to press the extension mechanism, the extension mechanism is extended vertically, protruding the top of the cue segment out of the billiard cue upper rack to facilitate removal.