Abstract: Starting from reducing the peak power and average power on the collector surface of a gyrotron oscillator, we have designed a magnetic field scanning system composed of 12 transverse elliptical AC coils and 2 longitudinal DC coils. According to the design parameters of the collector electrode of 140 GHz, TE_{28, 8} mode, 1 MW gyrotron oscillator, and combining electromagnetic field theory with particle-in-cell (PIC) simulation program, the scanning system can realize that the peak and average power density of the electron beam on the collector surface are not more than 404.91 W/cm² and 244.01 W/cm² respectively with a scanning length of 443.33 mm, which effectively alleviates the pressure of power dissipation and cooling of the collector.