Wellhead valve under-pressure replacement device and replacement method

The invention discloses a wellhead valve under-pressure replacement device and replacement method. The device comprises a pipe barrel, a sealing plug, a pressure relief opening, and a telescopic oil cylinder; the telescopic oil cylinder is installed at the other end of the pipe barrel; the telescopic oil cylinder is connected with a manual pump connector; a piston rod of the telescopic oil cylinder can be driven by the manual pump connector to move frontwards and backwards; a hole channel is formed in the piston rod; the hole channel is connected with the other pipeline of a manual pump; the sealing plug is installed at the front end of the piston rod and comprises a guide head, a connecting rod, a rubber sleeve and a piston shell; the head of the guide head is conical; the head of a slender rod of the connecting rod is connected and fixed with the guide head through threads; the rubber sleeve is connected between the guide head and the piston shell in a sleeved mode; the piston shellmoves to extrude and loos