Abstract: A computer-controlled automatic test platform was developed for evaluation of the open resonant cavity operating in high-order body mode of a gyrotron oscillator.The lab-built test platform comprises:host computer,mobile 3 D orientation manipulator,standard rectangular waveguide antenna,vector network analyzer(VNA),step-motor and control unit.The scanning precision and RF signal dynamic range were optimized to be 0.05 mm and of~100 dB,respectively.The field distributions,of the 140 GHz TE_28,8 high-order body mode resonant cavity,were experimentally evaluated with the test platform and numerically simulated.The measured and simulated results were in good agreement.For example,scanning at 0.5 mm X-step and 0.3 mm Y-step,the field crest-rough difference in Y-direction was measured to be about 10 dB.