The invention relates to the field of automobile gearbox test, and specifically relates to a gearbox no-load test inertia loading mechanism. The mechanism comprises an inertia flywheel, a translation slide, a drive shaft, a butt joint and a brake mechanism, wherein the inertia flywheel and the drive shaft are mounted on the translation slide, the two sides of the inertia flywheel are equipped with an inertia flywheel shaft, the inertia flywheel shaft on the front side of the inertia flywheel is connected with the drive shaft, the end, away from the inertia flywheel, of the drive shaft is connected with the butt joint, the front end of the butt joint is equipped with an applying pin which is used for being in butt-joint with a gearbox to be detected, the butt joint is driven by the translation slide to move, a spring for buffer is arranged between the drive shaft and the butt joint, the inertia flywheel shaft on the rear side of the inertia flywheel is connected with the brake mechanism, and the inertia flywheel starts or stops rotating under control of the brake mechanism. During no-load test of a gearbox, an external inertia can be applied to the gearbox under test according to actual circumstances.