The invention belongs to an automobile performance testing apparatus, in particular to an automatic fine tuning variable inertia moment type flywheel system. The system mainly comprising a half axle tube device (A) at a flywheel radius adjustment end, a flywheel device (B), a half axle tube device (C) at a force transmission end and a supporting device (D) is characterized in that: the half axle tube device (A) at the flywheel radius adjustment end and the half axle tube device (C) at the force transmission end are fixedly connected with the two sides of the flywheel device (B) and are supported by the supporting device (D) together with the flywheel device; the half axle tube device (A) at the flywheel radius adjustment end provides an adjustment drive force for the flywheel device (B), the flywheel device (B) changes the inertia moment of the flywheel through the variation of the radius of a circle consisting of split type mass blocks, and the half axle tube device (C) at the force transmission end realizes the input of the drive force. The automatic fine tuning variable inertia moment type flywheel system overcomes the defects such as the prior inertia flywheel set system enables only simple grading and fails to realize stepless adjustment with lower simulation precision, thereby being not adapted to testing small automobiles.