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.