Abstract: To know the effect of food on the population of Microtus fortis, the authors studied the feeding of young Microtus fortis on Carex brevicuspis, for providing a foundation for understanding the population growth of M. fortis, and references for the comprehensive management of M. fortis. C. brevicuspis was the main food of M. fortis in beach habitat of Dongting Lake, and was fed to the young M. fortis of 20-30 days of ages and different genders under the condition of artificial feeding. The results showed that: the weight of young female M. fortis decreased significantly (P<0.05), while that of the young male M. fortis increased slightly (P>0.05) after a week of feeding C. brevicuspis; meanwhile, the unit intake and unit excretion of young female M. fortis were higher than that of young male M. fortis, but the differences between them were not obvious (P>0.05). The study concludes that, the requirements for environment and food of young female M. fortis are higher than that of young male M. fortis at the stage of growth and development, single food can cause malnutrition in young female M. fortis.