The invention discloses a high-energy-efficiency piezoelectric ultrasonic transducer and an end cover thereof. The whole end cover is arranged to be of a cylinder structure and comprises an end cover front, an end cover middle and an end cover tail which are arranged coaxially. The end cover front is provided with an output end face. The end cover tail is provided with a fixing part. The end cover middle is of a combined hollow structure and comprises a cross-section transition section. The cross-section transition section is in a circular truncated cone shape or a right trapezoidal platform shape. A large end with the large area and a small end with the small area serve as the two ends of the cross-section transition section. The large end of the cross-section transition section is connected with the end cover tail. The small end of the cross-section transition section is connected with the end cover front. The outer wall of the cross-section transition section is supported on the end cover front by means of a plurality of rib plates uniformly distributed in the peripheral direction. Thus, by reasonably designing mass distribution of the end cover of the transducer, the mass load of the output end face is reduced, in comparison with a traditional solid end cover, by means of the overall arrangement, under the same input power, the amplitude of the output end face can be increased, and the output efficiency is improved beneficially.