Abstract: An independently tunable cells(ITC) thermionic cathode radio frequency (RF) gun is used as the injector for a compact FEL terahertz source in CAEP, and it is driven by microwaves through dual feed-in ports. One microwave feed-in port is located at the first cavity to excite the cavity and draw electron out from the cathode surface. Another is located at the last cavity to excite the rest cavities through coupling holes, There is no electromagnetic coupling between the two microwave feed-in ports. With regard to this kind of microwave working, the phase and amplitude of the electric field in the first cavity can be adjustable independently. Therefore the power of the back electron bombardment of the RF gun can be optimized and decreased by means of experimental research. Further more, higher quality beam can be obtained. The updated results about the power test on the thermionic cathode RF gun is introduced in this paper. A beam intensity about 400 mA is measured by beam-current transformer at the exit of the RF gun. And the effect is studied that back electron bombardment could be changed with adjustment of phase and amplitude of the electric field in the first cavity. The test results and phenomenon agree well with the theoretical design ones.