The utility model discloses a length extensible SF6 density tip check -up high -speed joint test wire, including high -speed joint test wire and extension test line, the SF6 density tip plug that the high -speed joint test wire is connected including the order, first multicore cable and extension circular connector are public mouthful, female mouthful extension circular connector, second multicore cable and check gauge contact access plug that the order is connected are drawn together to the extension test solenoid, structure that the check gauge contact inserted the plug with the structure that extension circular connector is public mouthful is the same, public mouthful of extension circular connector with female mouthful of common extension circular connector that constitutes of extension circular connector, the extension circular connector core -number with the core -number that the core -number of first multicore cable, the core -number of second multicore cable and check gauge contact inserted the plug is the same. The utility model discloses simple and convenient, experimental weak point consuming time, the extensive popularization and application of being convenient for are hanged down, are controlled to simple structure, risk.