The relevant facts to phase of relativistic klystron amplifier (RKA) are analyzed in this paper. The output microwave phase of RKA relevant to voltage, current, size, rising time and delay time of electron beams is simulated by PIC. It is found that the phase shift of RKA is changed with the voltage, current and size of electron beams. On the contrast, the leading magnetic field, rising time and delay time of electron beams do not change the phase shift of RKA obviously. The phase of RKA is investigated in experiment too. The phase sensitivity of RKA is about 2.6 degrees; the phase shaking of RKA is less than 20 degrees. The experimental results are content well with the simulations.