The flexural parasitic mode is the key technical bottleneck restricting the energy transmission efficiency and blade displacement output of the ultrasonic scalpel. To address this issue, this paper proposes a composite waveguide rod design that integrates an acoustic black hole (ABH) with a phononic crystal (PnC) structure, aiming to suppress the flexural parasitic mode and enhance the mechanical output capability of the scalpel. First, an ABH power-law profile is embedded into the design of a bamboo-shaped periodic variable cross-section rod. The effects of the ABH parameter m, minimum cross-sectional radius r₀, and the lattice constant a on the vibration bandgap distribution are systematically investigated through transfer matrix modeling and finite element simulations. When m = 4.1, r₀ = 0.85 mm, and a = 16 mm, the composite waveguide rod forms a Bragg bandgap for the flexural vibration near 55.5 kHz (bandwidth ~27 kHz, attenuation up to 30 dB), while the longitudinal vibration remains within the passband. Subsequently, the ABH-PnC composite waveguide rod is integrated into an ultrasonic scalpel, and an electromechanical equivalent circuit model that enables rapid prediction of frequency characteristics (error 1.5%) is established. Finite element analysis shows that the new scalpel exhibits only a single resonance peak near 55.5 kHz, with all other spurious impedance peaks completely eliminated, effectively suppressing the generation of the flexural parasitic mode. Furthermore, compared with a conventional scalpel using a uniform rod, the ABH-PnC scalpel achieves a 4.2% increase in blade longitudinal displacement and an 82% reduction in the flexural vibration amplitude of the rod and transducer. This study physically realizes the effective blocking of the parasitic flexural mode, providing a new theoretical pathway and an efficient design tool for the waveguide design of high-precision, high-efficiency ultrasonic surgical instruments.