Environmental factors play an important role in the seasonal adaptation of body mass, energy intake and thermogenesis in small mammals. The Hengduan Mountains region is at the boundary between the Palaearctic region and the Oriental region. It is alpine with high mountains and gorges; the diversity and abundance of mammals is high and it is considered to be "the harbor in fourth ice age". Therefore small mammals may differ from those from other regions. Apodemus is the most common rodent to inhabit the broadleaf forests in the Temperate Zone of the Palaearctic region; in China Apodemus chevrieri is distributed in the Hengduan mountains region, and on vales, it is the host of rat epidemic disease in the Hengduan mountains region. Evaporative water loss and energy metabolism in A. chevrieri were reported in 2008.The effect of cold acclimation on energy metabolism and body mass regulation in A. chevrieri was also studied in 2011, A. chevrieri decreased body mass and increased thermogenesis and energy intake. Nowadays, to investigate the roles of photoperiod and high fat diet in the regulation of energy budget and thermogenesis in small mammals in the Hengduan mountains region, 32 adult female A.chevrieri were acclimatized to one of 4 regimens, 1) long day and low fat diet, LL; 2) long day and high fat diet, LH; 3) short day and low fat diet, SL; 4) short day and high fat diet, SH. The animals in any groups were housed individually throughout the experiment. After a 7-week acclimation period, we determined body mass(Body mass was measured before the experiment and every 3 days throughout the acclimation of 7 weeks), energy budget (Dry matter intake, DMI; Gross energy intake, GEI; Digestive energy intake, DEI; Digestibility), basal metabolic rate (BMR), nonshivering thermogenesis (NST), body composition(Heart, Liver, Spleen, Lung, Kidneys, Wet carcass mass, Dry carcass mass, Body fat, Brown adipose tissue and so on), serum leptin level and body fat content. The results showed that short days induced decreases in body mass, wet and dry carcass mass, wet lung mass and serum leptin level, and increases in wet spleem mass and nonshivering thermogenesis. There were no significant effects of diets varying in body mass, basal metabolic rate, nonshivering thermogenesis and serum leptin level. However, high-fat diet significantly reduced dry matter intake, gross energy intake, digestive energy intake and elevated apparent digestibility and body fat content. Serum leptin level were positively correlated with body fat mass, however, not with energy intake. Our results suggest that A. chevrieri resist high-fat diet induced obesity, which might be mediated by the increased sensitivity of leptin and enhanced thermogenesis during short photoperiods. A. chevrieri in the field, can prevent excessive obesity by adaptive regulation of energy metabolism and thermogenesis.