Multisite task scheduling plays more and more important role in the grid computing as the WAN becomes faster and faster. Through the development of a three-level architecture of the distributed computing grid model and a grid schedule model, a scalable environment for multisite task scheduling is put forward. Then, a multisite Distributed Scheduling Server is designed and its prototype is implemented. A heuristic strategy, Clustering-based Grid Resource Selection algorithm, is described. Experiment indicates the scheduler and the algorithm are effective.