Abstract: The gyrotron traveling-wave-tube amplifier (Gyro-TWT) is capable of high frequency, broad bandwidth,and high power operation. It has become one of the most promising microwave source candidates for the millimeter-wave high resolution imaging radar of the next generation. This paper introduces the present state of gyro-TWT development in detail,comments on a series of key techniques related, and reveals the direction of its future trend, which is of importance for the domestic development of the next generation high power millimeter-wave microwave sources.