Integrated optomechanical crystal (OMC) cavities provide a vital device prototype for highly efficient microwave to optical conversion in quantum information processing. In this work, we propose a novel heterogeneous OMC cavity consisting of a thin-film lithium niobate (TFLN) slab and chalcogenide (ChG) photonic crystal nanobeam coupled by a wavelength-scale mechanical waveguide. The optomechanical coupling rate of the heterogeneous OMC cavity is optimized up to 340 kHz at 1.1197 GHz. Combined with phononic band and power decomposition, 17.38% energy from the loaded RF power is converted into dominant fundamental horizontal shear mode (SH0) in the narrow LN mechanical waveguide. Based on this fraction, as a result, 3.51% power relative to the loaded RF energy is scattered into the fundamental longitudinal mode (L0) facing the TFLN-ChG heterogeneous waveguide. The acoustic breathing mode of the heterogeneous OMC is successfully excited under the driving of the propagating L0 mode in the heterogeneous waveguide, demonstrating the great potentials of the heterogeneous piezo-optomechanical transducer in high-performance photon–phonon interaction fields.