High-order Hermite-Gauss modes are transformed from fundamental Gauss mode using mode converter cavity by mode-mismatching technique between input beam and cavity. Based on the nondegeneration of resonant cavity lengths and output amplitudes of different order Hermite-Gauss modes, an auto-searching and auto-locking servo system of cavity for high-order mode generation is realized and stable Hermite-Gauss modes of TEM00, TEM10 and TEM20 are obtained experimentally.