High-speed CNC machine is a kind of important scientific and industrial equipment. The spindle is a key component of the machine and the automatic balancing is important. An experiment platform has been built to test the properties of an electromagnetic ring balancer. Balancing test which uses influence coefficient method, mainly analyses from the following three aspects: Vibration test on platform itself; effects of rotating speed; effects of the angle of test weight. The experiment used an advanced automatic balancer and classical influence coefficient method. The results show the effects of platform and spindle speed, also get a simple way to get a good influence coefficient. A foundation has been built for two-plane balancing and modal analysis.