The grazing activities of ungulates are closely related to their own energy expenditure and the pasture status, but few studies have considered the mechanism under mixed grazing in desert steppe. Here, we statistically analyzed the spatio-temporal distribution pattern of ungulates' grazing activities by equipping GPS collars with triaxial acceleration sensors on mixed grazing cattle and sheep. Then, based on the spatial distribution and seasonal changes of pasture vegetation, a structural equation model of ungulates' grazing characteristics and energy expenditure was established, and the response model of mixed grazing cattle and sheep to grassland resources in desert steppe was discussed. The results showed that the cattle's daily grazing ODBA, grazing time and range increased over time, while the sheep's decreased. Both cattle and sheep showed spatial and temporal differences in daily grazing activity on pasture, and the differences between plots (spatial) tended to be more significant than the differences between months (temporal). The grazing activities of cattle and sheep on mixed pasture in desert steppe were influenced by vegetation cover (NDVI) as well as by the composition of the vegetation species, especially the distribution of poisonous forages (Ephedra sinica). This study provides a new idea for the mixed grazing of ungulates and the sustainable utilization of grassland resources in desert steppe, and suggests that human management should be used to intervene in the distribution of poisonous forages on pastures.