Abstract: This paper studies the seasonal variation of the age structure of the long tailed hamster population in the southwest of ecotone between agriculture and animal husbandry in Yanmenguan. The skulls of long tailed hamsters were collected and made into specimens according to the month. The age of the left upper molar of the skulls was mainly determined by the growth and wear conditions. The population of long tailed hamsters was divided into five age groups, and the age composition of the population of long tailed hamsters was analyzed by Ridit. It was found that there were no juvenile rats in the Cricetulus longtail population in December, and there were no juvenile rats and sub adult rats in the population from January to March. In the Southwest of ecotone between agriculture and animal husbandry in Yanmenguan, the young hamsters of the long tailed hamster appeared in early April to early November, which indicated that the breeding period of the long tailed hamster was longer.