The advantage of a fisheye lens is that it has a very large field of view and can provide more information. Nowadays, fisheye lenses are widely used in security, photography, aerospace, etc., especially in military and national defense. This paper firstly summarizes the imaging principle and development of fisheye lens, then analyzes the key issues, such as overflow of light or total reflection, and existing methods in the initial structure designing of modern fisheye lens, introduces the application and development trend of modern fisheye lens, and finally discusses recent crucial designing problems and the future of fisheye lens.