One of the main scientific aims of LHAASO is to measure the energy spectra of cosmic rays for individual mass compositions. LHAASO is made up of three detector arrays, which are WFCTA, KM2A, and WCDA. The three detector arrays can achieve hybrid observation, so several extensive air shower observables that are sensitive to mass compositions can be measured simultaneously. ROOT-TMVA package is used to combine these component-sensitive parameters for selecting proton events. The selected proton events have a purity of over 90%. In this paper, we have selected simulated events with shower cores located in the KM2A array to investigate the feasibility of the analysis method for studying the proton energy spectrum. We have also studied the associated systematic uncertainties, including those introduced by the composition models, proton selection, energy reconstruction method, and hadronic interaction models.