Because of the nonconservation of energy in the Rowe’s model for the large signal interactionin TWTs. The Rowe’s model produces considerable error in the TWT design, especially, in thedesign of helix TWT with velocity jump. A modified model which is based on conservation of energyis suggested to improve the accuracy of the design of TWT. By the new model, when the loss andthe space-charge are neglected, the energy got by the circuit is accurately equal to that given by theelectron beam. And all results obtained by the new model are more in agreement with the actualcases than those by Rowe’s model. Because of the nonconservation of energy in the Rowe’s model for the large signal interaction in TWTs. The Rowe’s model produces considerable error in the TWT design, especially, in the design of helix TWT with velocity jump. A modified model which is based on conservation of energy is suggested to improve the accuracy of the design of TWT. By the new model, when the loss and the space-charge are neglected, the energy got by the circuit is accurately equal to that given by the electron beam. And all results obtained by the new model are more in agreement with the actualcases than those by Rowe’s model.