The invention discloses a fixture for machining a valve ball stem and an application method of the fixture. The fixture comprises a pressing plate, four pull rods, tightening bolts, a ball seat, a connecting disc, tightening screws, connecting rods, a positioning block, a positioning rod and a positioning bolt. The pull rods penetrate the pressing plate to be connected with the tightening bolts, the ball seat is fixed on the connecting disc through the tightening screws, the pull rods are connected with the connecting rods through hinges, and the positioning rod penetrates the positioning block to be connected with the positioning bolt. For balls different in diameter, centering of four points of the four pull rods can be guaranteed by changing the positioning block in position and adjusting the pull rods in position, so that centering of the ball can be guaranteed; a new fixture is not required for the balls different in diameter; the fixture which is widely used is applicable to the balls ranging from 500mm to 1000mm, so that production cost is greatly saved.