Abstract: The ultrafast electron diffraction (UED) facility located in Shanghai Jiao Tong University, driven by a linear electron accelerator, has a photocathode RF gun. Sometimes an RF gun arc might happen during the accelerator running, causing a cavity detuning and beam loss, then resulting in a beam energy change. It will take a long time for the beam to restore its previous energy, which will influence utilization of the facility. An energy feedback is applied to low level RF (LLRF) system after improvement of amplitude-phase loop, using a real-time feedback of the beam center position to regulate the output amplitude of LLRF, to ensure the stability of beam energy and RF gun accelerating field. A long period of stability testing indicates, that beam energy can return to its original value quickly after arc occurence, energy jitter is improved from 4.293 3×10⁻⁴ (RMS) to 2.855 7×10⁻⁴ (RMS), realizing a long term stability of beam energy.