Abstract: In this paper,the formulae of power flow between a three series coupled oscillators have been derived.It is shown that power flow between the three series coupled oscillators is different from that between the two coupled oscillators,and not only includes the direct power flow,which exists in the two coupled oscillators,but also the indirect power flow between the indirectly coupled oscillators,which does not exist in the indirect coupled oscillators for classical Statistical Energy Analysis because of an assumption of weak coupling between the coupled subsystems.The indirect power flow is proportional to the differences among the stored energies in the indirectly coupled oscillators,and the amplitude of the power flow in different directions is the same,the direction opposite.The relationships between the indirect power flow and the parameters of the oscillators have been investigated using numerical method,and it has been shown that in strong coupling conditions the indirect power flow have to be considered.