Functional coatings are effective for improving the properties of devices. The development of thin-film-based surface acoustic wave (SAW) devices coated with piezoelectric zinc oxide (ZnO) thin films has attracted considerable attention. The orientation and thickness of piezoelectric films will affect the characteristics of SAW devices. In this research, (100) ZnO films were combined with diamond to form new composite SAW substrates. The SAW properties of (100) ZnO films on diamond with four composite structures, namely, interdigital transducer (IDT)/(100)ZnO/diamond, (100)ZnO/IDT/diamond, IDT/(100)ZnO/metal/diamond, and metal/(100)ZnO/IDT/diamond were systematically analyzed and some excellent SAW properties were observed. In particular, for mode 1 of (100)ZnO/IDT/diamond, a maximum K2 of 5.85% and a maximum velocity of 8857 m/s were obtained at a minimum film thickness ratio of h/λ = 0.11. Such research results provide a predictable theoretical basis for the further application of such substrates in high-velocity SAW devices.