Air-photos can be used for aiding various investigations. It is due to the special characteristics of air-photos themselves and their high relevancy to various field of study. So far air-photos are mostly used for studying physical problems. The following paper is to introduce the use of air-photos in the framework of studying urban population density. By employing air-photos as auxiliary tools of analysis, urban population density can be more clearly presented and can be divided into three types. The first type is commonly used in urban study, while the second and the third type are rarely presented. Based on the survey carried can give clearer and representative picture about the study area. Consequently, the second and the third concept of population density are really recommended in any urban study.