In this work, we studied the pH-dependent long-range ET catalyzed by lignin peroxidase by using the QM/MM MD simulations. In general, we focused on two ET steps in the catalytic cycle, with the first ET step involves ET from Trp171 to Cpd I and the second ET step involves ET from the substrate to Trp171. In the absence of the pH buffer of tartaric acid, our simulations indicated that the ET from Trp171 to Cpd I follows a stepwise ETPT (ET is followed by PT) mechanism, during which the imidazole nitrogen proton in Trp171 transfers to the adjacent base of Glu250, yielding a neutral Trp • radical (Scheme 2A). For the following ET from the lignin substrate to the neutral Trp • radical, our results suggested the reaction follows a concerted PCET mechanism. In contrast, when a tartaric acid molecule is added and forms a strong H-bond with Glu250, it can dramatically change the ET mechanisms for both ET steps. For the ET from Trp171 to Cpd I, our QM/MM-MD simulations showed the reaction transpires via a pure ET mechanism, leading to a stable Trp-H •+ cation radical. This is because the strong H-bond between Glu250 and the tartaric acid would significantly undermine the basicity of Glu250, preventing the PT from the Trp-H •+ cation radical to Glu250. Notably, the Trp-H •+ cation radical has a significantly higher electron affinity than that of the neutral Trp • radical. Hence, compared to the neutral Trp • radical formed in high pH conditions, the Trp-H •+ cationic radical formed in low pH conditions provides a significantly larger driving force for the one-electron oxidation of the lignin substrate.