A portable carbon dioxide adapter system, comprising: a first collar assembly for connecting to a liquid carbon dioxide source; a gasifying/ flow adjusting assembly that converts liquid carbon dioxide into gaseous carbon dioxide and is configured to adjust gas flow; an injection nozzle valve assembly so as to inject the gaseous carbon dioxide into a compressed gas gun; and a second collar assembly with one end slidably receiving the injection nozzle valve assembly and the other end being securely connected to the gasifying/ flow adjusting assembly, wherein an opening unit for piecing a disposable carbon dioxide gas cylinder as the liquid carbon dioxide source is mounted within the first collar assembly; and the gasifying/ flow adjusting assembly has a volume-adjustable hollow inner cavity configuration, characterized in that the injection nozzle valve assembly comprises a nozzle valve rod, an embedded sealing ring, an end sealing collar, an end locking cap, and a tension spring, wherein the nozzle valve rod slidably passes through a central through-hole of the second collar assembly, one end of the nozzle valve rod being a hollow nozzle hole, the other end thereof being a solid screw; in a peripheral direction of the nozzle valve rod is provided an annular groove receiving the embedded sealing ring, and meanwhile in a diameter direction of the nozzle valve rod is provided a gas inflow hole in communication with the hollow nozzle hole; compared with the gas inflow hole, the annular groove is disposed closer to the hollow nozzle hole side; the end sealing collar is mounted on the solid screw of the nozzle valve stem and is secured through the end locking cap. a first collar assembly for connecting to a liquid carbon dioxide source; a gasifying/ flow adjusting assembly that converts liquid carbon dioxide into gaseous carbon dioxide and is configured to adjust gas flow; an injection nozzle valve assembly so as to inject the gaseous carbon dioxide into a compressed gas gun; and a second collar assembly with one end slidably receiving the injection nozzle valve assembly and the other end being securely connected to the gasifying/ flow adjusting assembly, wherein an opening unit for piecing a disposable carbon dioxide gas cylinder as the liquid carbon dioxide source is mounted within the first collar assembly; and the gasifying/ flow adjusting assembly has a volume-adjustable hollow inner cavity configuration, characterized in that the injection nozzle valve assembly comprises a nozzle valve rod, an embedded sealing ring, an end sealing collar, an end locking cap, and a tension spring, wherein the nozzle valve rod slidably passes through a central through-hole of the second collar assembly, one end of the nozzle valve rod being a hollow nozzle hole, the other end thereof being a solid screw; in a peripheral direction of the nozzle valve rod is provided an annular groove receiving the embedded sealing ring, and meanwhile in a diameter direction of the nozzle valve rod is provided a gas inflow hole in communication with the hollow nozzle hole; compared with the gas inflow hole, the annular groove is disposed closer to the hollow nozzle hole side; the end sealing collar is mounted on the solid screw of the nozzle valve stem and is secured through the end locking cap.