A method for improving the uniformity of high-frequency discharge plasma by means of frequency modulation is disclosed. In a plasma discharge chamber, there is a pair of parallel electrodes. A high-frequency power supply is adopted to feed the electrodes. The frequency range of the electromagnetic field is 13.56 MHz˜160 MHz. Discharge gas is input to form plasma. The frequency of the fed-in high-frequency electromagnetic field is under automatic tuning control, and keeps changing cyclically without stop in the course of plasma discharge. The range of the frequency change may fall into either a portion of or the entire range of 13.56 MHz˜160 MHz and makes the locations with higher plasma density on the plane in parallel with the electrodes and in the plasma discharge space changed cyclically. In a time slot longer than one frequency change cycle, the average plasma density between the parallel electrodes is uniform.