A bar length detection tool and a method for detecting a length of a bar (14) are provided, the bar length detection tool comprises: a substrate (1), a first bearing seat (2) and a second bearing seat (2') oppositely disposed on the substrate (1), and a micrometer (3); wherein a positioning device (5) is rotatably connected to the first bearing seat (2), a mounting plate (6) is rotatably connected to the second bearing seat (2'), and the first bearing seat (2) and the second bearing seat (2') are each provided with at least two needle bearings (12).