By studying the trophic niche of rodents, we can not only understand the nutrient layer occupied by rodents in the ecosystem, but also reveal their main food sources and their relationships with changes observed in rodents at different growth and development periods. In this study, 81 rodent specimens of 4 species were captured using the snap-trapping method in typical grasslands of Inner Mongolia in July 2018. The trophic niche of Brandt’s voles (Lasiopodomys brandtii) and their interspecific relationship with other three common rodents, i.e., Spermophilus dauricus, Phodopus sungorus and Allactaga sibirica, were analyzed using ¹³C and ¹⁵N stability isotope techniques. We found a marked age-specific trophic niche in the width of bone tissue in Brandt’s voles, i.e., the Ⅲ age group had the widest with 0.802 ‰² Bayesian standard ellipse area,  followed by the Ⅱ age group with 0.699 ‰² ellipse area, and the lowest in the Ⅳ age group with 0.666 ‰².  The trophic niche overlap commonality area among the three age groups was 0.064 ‰², and common food sources accounted for less than 10% of all food sources. Moreover, we did not find that Brandt's voles competed for food sources with the other three rodents in either the short-term or long-term diet. However, compared with the other three rodent species, the niche width of Brandt's voles was narrower and its food selectivity was stronger. These findings suggest that if food source composition changed and was no longer suitable for Brandt's voles in their habitat, the vole might face food shortage and emigration would be a direct and favorable strategy for the development of the voles’ population.