We propose a mathematical model which can quantitatively describe the kinetics of postsynaptic membrane potential modulated by a low intensity laser. By virtue of the numerical computation, it is found that the height of peak of fast excitatory postsynaptic potential (f-EPSP) for intermediate laser power density (around 2mW/cm2) is higher than that for both small and large laser power densities, that is, of the f-EPSP peak height changes with the increasing laser power density (up to 5mW/cm2) in non-monotonic behaviour. The theoretical results are qualitatively consistent with the previous experimental data.