Based on the theory of open resonant cavity and quasi-optical mode conversion, a high-order quasi-optical mode generator was designed and manufactured for the cold measurement of the gyrotron oscillator. For the high-order mode generator working at 140 GHz and TE_28,8 mode, the corresponding experimental measurements were completed using the three-dimensional mobile measurement platform and the network analyzer which were controlled by computer programming. The results show that, when the simulation results related to the resonant frequency, Q factor, and the correlation of the electric field distribution with the TE_28,8 mode in the cavity are respectively 140.179 GHz, 855, and 90.9%, the corresponding cold-tested ones for resonant frequency and Q factor are respectively 140.155 GHz and 876, and the transverse electric field distribution tested is very similar to that of the TE_28,8 mode. It indicates that the quasi-optical mode generator designed in this paper can provide an effective experimental platform for the research and design process verification of the high-order body mode gyrotron oscillator.