With the steady aggravation of global warming, dairy cows face increasing heat stress, which results increasing economic loss to the dairy industry. Heat stress occurs when cows are exposed to extreme heat and cannot maintain their core temperature. Under hot temperature environment, dairy cattle is characterized by increased respiration, heart rate, and rectal temperature and decreased feed intake, which have a negative impact on endocrine system and immune system and seriously reduce milk performance of dairy cows. Previous studies of heat stress in dairy cows focused on its effects on productive performance, especially to alleviate the effect of heat stress by functional feed additives, but the specific mechanism of heat stress is not clear. This paper summarizes the widespread impact of heat stress on milk production and quality of dairy cows in different regions, and introduces the known mechanisms of heat stress from aspects of energy metabolism, endocrine, oxidative stress, apoptosis and autophagy. In the aspect of energy metabolism, heat stress induces a negative energy balance in cows by reducing food intake and inducing disorder in lipid breakdown and energy metabolism; In terms of endocrine system, heat stress affects the hypothalamic-pituitary-adrenal axis / thyroid axis / gonadal axis / growth axis of dairy cows and results in hormonal changes. In oxidative stress, heat stress affects the ROS levels in vivo and activates related defense signal pathways. In the aspect of apoptosis and autophagy, heat stress contributes to the damage of mammary epithelial cells by the expression of apoptosis-related genes and by excessive autophagy. The paper put forward that in the foreseeable future, heat stress will be a major challenge for the dairy industry. It is necessary to establish cows heat stress models to comprehensively investigate the basic mechanisms of heat stress in dairy cows using advanced cellular and molecular techniques. In addition, efforts should be placed to develop new interventions to reduce the damage of hear stress to dairy cows.