The phase property is an important parameter of the intense relativistic klystron amplifier (RKA).But the intense relativistic electric beams (IREBs) with a long rise time in terms of high-power pulse power source will cause the output microwave phase jitter when driving the RKA

which limits the RKA's applications.Based on the intense current pulse electron beam properties

the impacts on output microwave phase by the increase of the electron beams' energy in the front section of the pulse and the spurious frequency caused by energy change in the cavities are analyzed through the theory.And then some PIC simulations and experiments are studied.It is found that the increase of the electron beams' energy in the front section lead to the output microwave phase continually changing.Besides

the spurious frequency in the cavity caused by the front section of the electron beams

which lead to the output microwave phase continually jitter in the front and the early of the stable section of the pulse.In the condition of cavities' parameters are constant

the output microwave phase is significantly affected by the rate of electron beams energy in the front section:when the rate of electron beams energy increase

the amplitude of the phase jitter and the jitter duration increase.And the stabilization of the phase can be increased by reducing the Q of the cavity at the frequency point of the spurious frequency.