With the development of urbanization and motorization, the imbalanced contradiction of urban traffic between supply and demand becomes increasingly sharp. Traffic congestion has become very serious and results in problems such as increase of traffic accidents and rise of fuel depletion. Expanding existing links in the urban traffic network is one of the commonly used methods which can effectively solve the capacity limitation problem and is considered more sensible for road networks. Thus, it is important to evaluate the expansion effect of links, so as to meet the demand for the expansion of traffic network capacity, and the minimum cost of expansion. This study puts forward the definition of a link’s contributive degree to evaluate the expansion effect of links for traffic network capacity. According to the stochastic user equilibrium model, a bi-level programming model for determining the maximum contributive degree of link is proposed. Moreover, the efficient-paths-based stochastic traffic assignment algorithm is adopted based on the impact area and the evaluation process for contributive degree of links is put forward. In order to demonstrate the efficiency of the proposed model and algorithm, it is applied to a simple network and some results have been got.