Spring and summer rainfall patterns in Mongolian semi-arid grazing lands vary greatly from year to year, and are thought to affect the energy balance and nutritional conditions of sheep via plant community changes. To test this, climatic and vegetation data in Mongolian semiarid grasslands were obtained from spring to summer over 3 yr. For 2 yr, sheep energy intake and expenditure were calculated, and the energy balance and nutrition indicators were compared among years with different seasonal rainfall patterns. In 2019 and 2022, rainfall patterns were characterized by the presence and absence of early summer drought, respectively. Compared to 2019, plants were tall and abundant in 2022; thus, the mean bite size, energy intake, and body weight were higher. Estimated energy intake and expenditure were 4.56 and 2.10 Mcal in 2019, and 6.75 and 1.63 Mcal in 2022, respectively, indicating that rainfall timing and amount affected vegetation assemblage, plant height, herbage nutrition, grazing behavior, animal energy balance, and nutrition conditions. Insufficient rainfall before early summer in semiarid grazing lands resulted in low energy intake and balance, and delayed body weight gain could not be recovered. These results can inform the management of grazing lands to ensure optimal livestock conditions.