Ultrasonic metal welding is used to join metallic materials using an ultrasonic horn. When welding the terminal of a secondary battery with multilayered thin plates, a large area is required for the horn and tip owing to the wide welding area. Accordingly, the vibration amplitude is not structurally uniform. This causes non-uniformity in the welding strength, which deteriorates the welding quality. In this study, we attempt to improve the shape of an existing horn to enhance the amplitude uniformity of tips in a half-wavelength ultrasonic welding horn with a large-area tip. Through modal analysis, the resonance frequency of the horn and the vibration amplitude at five tip positions are obtained analytically. The amplitude uniformities of the modified and existing horns are compared and verified using a laser displacement sensor. Based on actual measurements of the amplitudes at five positions, the maximum deviations of the amplitudes of the existing and modified horn are 2.7 and 0.2 µm, respectively, and the standard deviations of the amplitudes are 1.005 and 0.075 µm, respectively, indicating that the amplitude uniformity of the modified horn improved significantly.