Thermoelectric materials are capable of converting waste heat into electricity. Half-Heusler materials, as one of the promising candidates for thermoelectrics, have a relatively low figure of merit due to their high thermal conductivity. Here, we propose an effective strategy to lower the lattice thermal conductivity of half-Heusler materials guided by the first principles calculations and Boltzmann transport equation. The strategy was inspirited by regulating the phonon dispersion with a combination of two heavy and one light atoms, which introduced the absence of acoustic-optic phonon gap (a–o gap) and promoted the phonon–phonon scattering phase space, and therefore a small lattice thermal conductivity. Taking half-Heusler LuNiBi as an example, we found that it possessed an ultralow lattice thermal conductivity (0.7 W m⁻¹ K⁻¹ at 300 K after two-channel phonon transport model correction), which was two orders of magnitude smaller than that of usual half-Heusler materials. Our findings provide an effective strategy to design half-Heusler materials with low thermal conductivities and serve as a guide for the further improvement of the thermoelectric performance of half-Heusler compounds.