The study of ultrasonic variable amplitude rods is an important part of ultrasonic vibration system. The cosine half?period cylindrical composite horn was designed to realize the smooth transition between the cosine section and the cylindrical section. By theoretical calculation, its frequency equation and amplification coefficient equation were derived. The results show that the frequency equation of the new cosine cylindrical composite variable amplitude rod is simpler than that of the cylindrical conical composite variable amplitude rod and the stepped variable amplitude rod; the amplification coefficient equation of the new cosine cylindrical composite variable amplitude rod is similar to that of the stepped variable amplitude rod, which is more concise than that of the cylindrical conical composite variable amplitude rod. Using ABAQUS finite element analysis software, modal analysis of the designed cosine cylindrical composite variable amplitude rod was carried out to determine its inherent frequency and vibration type; harmonic response analysis was conducted to check the feasibility of the theoretical design. The results of the modal analysis and harmonic response analysis show that the performance of the ultrasonic composite variable amplitude rod is optimal when the length of the cosine section is 55 and 60 mm. The research results provide a reference for the design and application of ultrasonic composite amplitude rod.