The demonstration of a 100 mA, 4.6 MeV superconducting radio frequency linear electron accelerator, based on conduction cooling and developed by the Institute of Modern Physics (IMP), aims to validate the feasibility of stable beam commissioning in a liquid helium-free 5-cell-$\beta$opt=0.82 Nb3Sn elliptical cavity, and to offer guidance for subsequent industrial applications. The beam energy characteristics, considered one of the critical parameters, need to be precisely measured. Given the high beam energy and the need for a compact, straightforward accelerator layout, we achieved high-precision measurements using only a ordinary dipole, a slit, and a Faraday Cup (FC). This paper presents the online measurement results of beam energy at different cavity voltage and provides a thorough analysis and optimization of the various errors encountered during measurement.