Phyllosphere contains a wide distribution of nitrogen-fixing bacteria that fix nitrogen for bacterial and plant growth. The properties of nitrogen-fixing bacteria have been partially studied through traditional culture-dependent methods. However, knowledge on nitrogen-fixing bacteria diversity, community structure and function in phyllosphere remains limited. Recently, developments in molecular ecology techniques have led to the widespread use of culture-independent methods, enable researchers to understand phyllosphere microbial diversity and ecological function. Phyllosphere nitrogen-fixing bacteria exhibit abundant diversity that is mainly affected by environmental conditions (temperature, humidity and light), the host plant species and microorganism interactions. Unlike rhizosphere diazotrophs, phyllosphere nitrogen-fixing bacteria may not be affected by chemical fertilizers and could thus be potentially beneficial in agricultural applications. This review discusses the latest research on phyllosphere nitrogen-fixing bacteria diversity and ecological functions in agricultural, forest, and marine ecosystems, and on the factors influencing the nitrogen-fixing system, thus providing new insight into phyllosphere nitrogen-fixing bacteria.