To efficiently adjust the output beam energy of the Hefei Light Source II (HLS-II) linac, this study presents a beam energy adjustment scheme. During the debugging stage, the beam bunch state is observed, and the beam energy is measured using an energy spectrum analysis system. In the storage ring injection stage, three Beam Position Monitors (BPMs) are employed for online beam energy measurement. An automatic phase scanning program is utilized to scan the output phase of the klystrons, deriving the energy gain formula for each acceleration section. By quantitatively adjusting the output phase and high voltage of the klystrons, rapid adjustment of the output beam energy of the linac is achieved. The online application results demonstrate that the proposed scheme can swiftly adjust the beam energy, with the adjusted beam exhibiting excellent quality and a transverse energy spread of less than 0.22%. Furthermore, the implementation of this scheme significantly improves the injection rate.