This paper proposes an analytical model for the blocking performance of adaptive routing over WDM networks with sparse wavelength conversion. Two key components of the model are 1) separate routes into segments, and calculate the blocking performance of each segment; and 2) calculate overflow traffic to wavelength convertible node, and then get the blocking probability of the node. The blocking probability of the whole network is then calculated as the combination of the blocking probability of segments and nodes. Based on the model, an adaptive routing algorithm is proposed, which is able to finish routing and wavelength assignment in one single step. Numerical results show that among the parameters considered, the number of wavelengths turns out to be the primary factor for the blocking performance.