The strategy of designing efficient room-temperature phosphorescence (RTP) emitters based on hydrogen bond interactions has attracted great attention in recent years. However, the regulation mechanism of the hydrogen bond on the RTP property remains unclear, and corresponding theoretical investigations are highly desired. Herein, the structure-property relationship and the internal mechanism of the hydrogen bond effect in regulating the RTP property are studied through the combination of quantum mechanics and molecular mechanics methods (QM/MM) coupled with the thermal vibration correlation function method. Intermolecular interactions, excited-state transition properties, reorganization energies, radiative and nonradiative decay rates, and the intersystem crossing rates are analyzed in detail. Results show that intermolecular hydrogen bonds can effectively delocalize molecular orbitals, enhance spin-orbit coupling (SOC) effect, and thus accelerate intersystem crossing (ISC) processes. In addition, an intermolecular hydrogen bond can also suppress nonradiative transition by restricting molecular motion, thereby promoting generation of phosphorescence. However, an excessively enhanced intermolecular hydrogen bond effect promotes molecular vibrations, leading to increased reorganization energies and thus facilitating nonradiative energy consumption process. The hydrogen bond "double-edged sword" effect on RTP properties and nonradiative decay process is theoretically revealed. Therefore, reasonable control of the hydrogen bond strength is beneficial for the development of efficient RTP emitters. Our research provides rational explanations for previous measurements and highlights the hydrogen bond effect in constructing efficient RTP emitters.