The reaction mechanism of long-range intramolecular electron transfer between the porphyrin-benzoquinone donor-acceptor pair linked by an organic spacer in acetonitrile solvent is investigated theoretically. The rate formula is derived on the basis of Fermi＇s golden rule for reactions induced by the through space and through bond type electronic couplings involving the dynamical effects of solvent fluctuation. Molecular dynamics (MD) calculations are carried out to construct the reaction free energy curves. The reaction rate is estimated on the basis of the results of MD calculations. It is found that there are important contributions from the vibrationally induced couplings by the solvent fluctuation. In order to examine the solvent fluctuation effect, we determine the effective electronic coupling element (ECE) and perform the decomposition analyses. The important electron pathway in determining the solvent-induced ECE is discussed.