Fall armyworm Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) is a destructive, migratory pest that seriously damages corn, sorghum, and other crops. The “insect killer” Pyemotes zhonghuajia (Trombidiformes: Pyemotidae) is an ectoparasitic mite of agricultural and forestry pests in China. To explore the biological control efficiency of P. zhonghuajia on S. frugiperda, we evaluated the parasitic selectivity of P. zhonghuajia on S. frugiperda of different life stages, and distribution of P. zhonghuajia on different body parts of S. frugiperda larvae and pupae, food intake of S. frugiperda larvae affected by P. zhonghuajia density, and field population dynamics of S. frugiperda in relation to the releasing population density of P. zhonghuajia. We found that P. zhonghuajia preferred 4th- to sixth-instar larvae and pupae of S. frugiperda for parasitism, and the ventral part of abdominal segments of fourth-instar larvae and the ventral part of pupal thorax for feeding. Freezing of S. frugiperda 4th instar larvae and pupae had little effect on the distribution of P. zhonghuajia on the surface of these two host stages. The parasitism of P. zhonghuajia regardless of mite population size (i.e., 5, 10, or 20 mites) could significantly decrease the cumulative food intake of fifth-instar larvae of S. frugiperda. Releasing two and three boxes (50,000 mites/box) of P. zhonghuajia significantly suppressed the field population of S. frugiperda on sorghum plants/25 m2 within 7 days. Our results show that P. zhonghuajia may be a potential biological control agent and can be used for biological control of S. frugiperda in the field.