Commercial street is a frequently-used urban space for citizen’s social contact and public life. Coexistence of traffic function and business function is the main situation of commercial street in old downtown area of Chinese cities. Its specific geographic location and function features results in great influences on urban structure, traffic condition and life quality of downtown area. However, as the rapid development of urbanization and economic growth in recent years, the commercial street in downtown area witnesses more and more problems resulted from the booming population, the prevalence of private vehicles, the springing up of commercial buildings and diversified demands of public spaces. Those problems are in urgently need of being solved through renovation design of commercial street. This thesis conducts the research on commercial street in Chinese old downtown area. It aims to do a deeply study on how theories of urban function, aesthetic expression and human behavior are applied to make assessments and guide the design of commercial street in Chinese old downtown area. To reach the research aims, the thesis elaborates the situations and problems of commercial street with explanation of the causes. Based on the problems, the theories of urban function, aesthetic expression and human behavior are studied and discussed. To implement these theories, Zhujiang Road in Nanjing has been chosen as a case, which is a typical commercial street with many problems in old downtown area. The assessment of this case based on the theories shows the serious issues of traffic congestion and disorder of cycle lane and sidewalk, bad walking environment and lack of public spaces. Follows the assessment is the design proposal for renovation, the surrounding area is replanned for a reformative traffic system and land-use, the street width is rearranged for better traffic environment, the public spaces and details of the street are redesigned into a rational and human-friendly commercial street. The thesis concludes by providing specific solutions to a case practice and summarizing series of solutions to the general problems of commercial street in Chinese old downtown area. It contributes to literature on problem-solving research of commercial street in Chinese old downtown area.