Abstract: With the high demand on the phase difference in dual-satellite interfering imaging of SAR, the phase change of power amplifiers becomes more important. The causes for phase flutter of power amplifier are analyzed based on the theory of microwave transmission line, the stress-strain of material is theoretically analyzed from the point view of engineering mechanics. The stress-strain of power transistor lead and SMA connector pin under the different temperatures is simulated by mechanics simulation software. The influence of grounding discontinuity and transient impedance changing of transmission line on transmitting phase caused by material stress is simulated by EM field simulation software. The result is obtained that the power amplifier (PA) with classic structure probably has the inherent characteristics of phase flutter, and it is verified by temperature cycling test. Finally several improving measures are offered for phase flutter and can be extended into the design of satellite borne PA.