Heat stress not only has different degrees of negative effects on the production and reproduction performance, immune system and product quality of ruminant, but also brings huge economic losses to animal husbandry. Selenium is one of the important trace elements, which has antioxidant and immune-enhancing effects, dietary selenium is helpful to alleviate heat stress in livestock and poultry. This article reviewed the evaluation criteria of heat stress and its effects on ruminant performance, immune system and rumen microbe, as well as the research progress on alleviating heat stress by dietary selenium supplementation, aiming to provide reference for reducing the harm of heat stress to ruminants and promoting the healthy and efficient development of livestock industry.