Most organisms have suitable climatic and ecological geographic distribution areas. The northern limit is attributed to low temperature, but some organisms exist southern limits in their geographic ranges. Because the development of insects is tightly correlated with environment, understanding the formation causes of insects' southern limits could provide theoretical basis for predicting their dispersal and outbreaks. In this article, the mechanisms and influential factors of insects' southern limits in the Northern Hemisphere were summarized from environment, hosts, and human activities. In the low latitude regions, southern limits of insects could be caused by the high temperatures during summer seasons by increasing insect mortalities, decreasing insect reproduction, and disorganizing insect symbionts, as well as caused by the excessive high minimum temperatures during winter seasons by destroying insect diapause rhythm. In addition, the distribution of hosts and human activities also affect the southern limits of insects. Climate warming reveals different impacts on the southern limits of different species of insects. The distribution of most insects will be expanded, while some insects limited by environments will move northward. The changes in southern limits of insects caused by global warming, and differences in southern limits of quarantine insect pests in their native and invasive ranges, as well as the reasons, should be further studied to provide supports for predicting the changes of insect distributions, and also provide novel approaches against insect pests.