This paper presents a new automatic compensation system of high tension reactive power，which consists of single-chip microcomputer，high tension capacitor and control device. The power factor measurment is solved by using discrete Fourier transform under the condition of a great deal of harmonics，and using nonlinear control technique，the automatic compensation of reactive power is performed. The measurment and compensation is accurate，and the life of high tension vacuum switch is long. It has been proved in practice that the performance of this system is stable and reliable，and its economical effect is remarkable.