A thermionic RF gun is a compact and economical electron beam system that is easy to operate. However, the electron back-bombardment of the cathode results in deterioration of beam quality and seriously limits the pulse duration. In this work, we investigate the use of time varying RF power into a thermionic RF gun in order to reduce the beam energy degradation by the back-bombardment effect during a macropulse. We have successfully increased the pulse duration from 3 μs to 4 μs and the total charge from 98 nC to 206 nC by monotonically increasing the RF power from 5 MW to 6 MW during the macro pulse. In addition, we calculated the RF power deposition using an equivalent circuit model. The beam loading was evaluated using one dimensional thermal conduction model. Good agreements with the experimental results were obtained for the beam energy and the RF power deposition during the macropulse.