Nitrogenous compounds have been extensively utilized as hydrogen atom transfer (HAT) catalysts in photoredox reactions, with nitrogenous radical cations being the actual hydrogen atom abstractors. Building upon our previous work, 120 thermodynamic graphs of nitrogenous radical cations abstracting hydrogen atoms, which encompass seven vital thermodynamic parameters, are designed and established to elucidate their redox characteristics. Furthermore, the applications of thermodynamic graphs to select appropriate photocatalysts, assess the feasibility of the HAT process, and diagnose the possible activation mechanism were discussed, which would enable the utilities of nitrogenous compounds as HAT catalysts or nitrogenous radical cations as hydrogen atom abstractors in photoredox reactions.