Because of explosive growth in Internet traffic and high complexity of heterogeneous networks, improving the routing and wavelength assignment (RWA) algorithm in underlying optical networks has become very important. Where there are multiple links between different the node pairs, a traditional wavelength-assignment algorithm may be invalid for a wavelength-switched optical networks (WSON) that has directional blocking constraints. Also, impairments in network nodes and subsequent degradation of optical signals may cause modulation failure in the optical network. In this paper, we propose an RWA algorithm based on a novel evaluation model for a WSON that has multiple constraints. The algorithm includes comprehensive evaluation model (CEM) and directional blocking constraint RWA based on CEM (DB-RWA). Diverse constraints are abstracted into various constraint conditions in order to better assign routing and wavelength. We propose using the novel CEM to optimize routing according to an assessed value of constraints on transmission performance. This eliminates the effects of physical transmission impairments in a WSON. DB-RWA based on CEM abstracts directional blocking conditions in multiple links between network nodes into directional blocking constraints. It also satisfies rigorous network specifications and provides flexibility, scalability, and first-fit rate for the backbone, especially in multiple links between WSON nodes.