The invention discloses a pre-bending tool for rear door hemming of a car body in white and a use method of the tool. The tool comprises a support stand pillar, a hemming lower die arranged on the support stand pillar, a four-bar mechanism and a drive mechanism, wherein the four-bar mechanism comprises a hemming pressing arm, a first connecting arm, a second connecting arm and a hemming arm corner seat, and a first end of the hemming pressing arm is hinged with a first end of the first connecting arm; the neighbouring part of the middle of the first connecting arm is hinged with the hemming arm corner seat; a second end of the hemming pressing arm is hinged with a first end of the second connecting arm; a second end of the second connecting arm is hinged with the hemming arm corner seat, and the hemming pressing arm, the first connecting arm, the second connecting arm and the hemming arm corner seat define a quadrangle; a pressing arm bulging outwards is further arranged on the hemming pressing arm, and a hemming upper die matched with the hemming lower die is arranged on the pressing arm; the drive mechanism drives the second end of the first connecting arm to move, a rear door is pre-bent by a large angle about 30 degrees and then conveyed to a press fit die for press fit, the cost is reduced, and the hemming efficiency and quality are improved.