On the base of Graph Theory, free-scale structure viewpoint and vertex degree indices, the paper describes and analyzes numeral and spatial characteristics and their urbanization relationships of drainage structure within the Lake Taihu watershed. Numeral degree of Nanhe drainage structure in Lake Taihu watershed does not accord with power function distribution, but length and rank degrees represent the characteristics of scale-free structure. Additionally Nanhe drainage structure is appropriate for not only Horton First Law and Second Law but also scale-free structure. Scale-free structure really fits a power function, which reflects an imbalance-structure system controlled by few gathered nodes and provides some key methods for identifying and modeling hydrology index of watershed. Lastly the power law behind the drainage structure shows the human disturbance difference exerted on drainage structure. In the highly urbanized area of Nanhe middle stream and downstream numeral and rank vertex degrees are both high, but length degree is low and in the less urbanized area, the area rank vertex degree is also very low. In the upstream area, urbanization level is low and length vertex degree is high, but rank vertex degree is low. The numeral vertex degree of most of nodes is low and has no clear relationship with urbanization.