Active measuring controller is applied in internal grinder to improve the grinding precision of bearing bore diameter. A novel active measuring controller was designed with linear operational amplifier and transistor as components. Oscillator circuit converted the workpiece dimension variation detected by inductive transducer into electric quantity. Rectification filter circuit converted the AC signal into DC signal; DC output signal was divided into four circuits. One was sent to level comparison switching circuit with highlow precision range; one was sent to indicating circuit. One was sent to transmission circuit. And one was taken as linear compensation network. The designed active measuring controller circuit boasts reliable performance in practice. The grinding precision of bearing can reach 1 μm.