Abstract: Although fractal theory provides a new method and means to study a number of complex natural phenomena，there is not constant dimension fractal manifestations strictly exist in the nature． A large number of complex phenomena needs to use variable dimension fractal to describe，and fractal characteristic of a river length is no exception． By using of variable fractal dimension to obtain the law of the length in section of Chongqing city of Yangtze River，the results conclude that: It does express a second-order accumulated variable-dimensional fractal phenomenon． And the dimension can reflect t the degree of bending of the river，the greater dimension，the more the river bend． It has different dimensions at a different location in the same river． In the same river，the larger dimension，the worse flow discharge capacity of the river，and the more obvious of the flood will be on the performance．

Key words: fractal dimension, variable dimension fractal, flood

摘要： 应用分形理论对长江重庆主城段进行了分形标定。研究表明: 长江重庆主城段河流长度具有二阶分维特性; 河流长度分维数可以反映河道的弯曲程度，分维数越大，河道越弯曲; 在同一河段，河流长度分维值越大，河流的泄 洪能力越差，洪水发生的可能性和洪水的强度越大。

关键词: 分形维数, 变维分形, 洪水