The invention relates to a medical electron accelerator technology and especially relates to a microwave excitation source with a self-fixed amplitude technology used for a high-energy medical electron accelerator. The microwave excitation source comprises a linear power supply used for system power supply, a reference oscillator, a phase-locked source, an electric control attenuator, a modulation switch, a preamplifier, a high power amplifier, an isolator, a power coupling unit, a power monitoring module II and a control mainboard, wherein the reference oscillator, the phase-locked source, the electric control attenuator, the modulation switch, the preamplifier, the high power amplifier, the isolator, the power coupling unit, the power monitoring module II and the control mainboard are successively connected. The control mainboard comprises an A/D and D/A module group and a main controller MCU. A signal output terminal of the power monitoring module II is connected to the A/D and D/A module group. A control output terminal of the A/D and D/A module group is connected to the electric control attenuator. A communication port of the A/D and D/A module group is connected to the main controller MCU. The main controller MCU is connected to a display unit. The modulation switch is connected to a time sequence controller. In the invention, through an automatic control unit, real-time power amplitude control is performed so that excitation source output power can be monitored in real time and be automatically calibrated.