The energy extraction of two flapping foils with tandem configurations has been studied based on velocity potential theory. The interactions of the hind foil and the vortices of the fore foil with typical oscillatory motions are investigated. The global phase shift, which combines the longitudinal distance and the phase shift of the motions of the two foils, is introduced as the indicating parameter of the interaction modes. The effects of longitudinal distance and phase shift of the motions on the energy extraction efficiency have been analysed. The most favourable global phase shift and the corresponding vortex pattern have been investigated.