To describe the vital characteristics of the urban ecosystem, the analogy of urban vital organism is introduced in this paper to vividly and systematically assess the urban ecosystem evolution. The framework of urban vitality index including productivity power, living status, ecological ascendancy and vital force, which respectively represents the situation of urban economic subsystem, social subsystem, natural subsystem and ecological regulatory subsystem, is also constructed. Accordingly, the concrete index system, model and gradation standard are proposed to assess the urban vitality, with the Wanzhou district of Chongqing city being chosen as a case study. The results indicate that the urban vitality indices of Wanzhou district during 2000-2005 are in the relatively weak grade with gradual improvement. It is therefore suggested that ecological planning measures on productivity power, living status, ecological ascendancy and vital force be implemented, in which the promotion of vital force of the urban ecosystem should be emphasized, to improve the urban vitality.