Abstract: With the development of service computing, more and more sources are released and utilized as services. Competition between service providers grows increasingly fierce. Hence, the win-win cooperation between services becomes an inevitable trend. Moreover, the consideration of the quality constraint correlation between businesses further complicates the service composition optimization problem. To solve it, this paper uses task granulation on service composition business process, and presents a quality constraint-aware service composition method based on task granulation (Tg-QcA) when considering the quality constraint between candidate services. Firstly, this paper makes theoretical analysis and verifies that each QoS aggregation has component mode and the utility function of multi-attribute service composition problem still has component mode, thereby guaranteeing the completeness of task-granulation optimization method. Secondly, a quality constraint based model is built and then a task granulation partition is made through the subjection degree between tasks to decompose the original problem, thereby reducing the solving scale of problem. Finally, it is demonstrated by computer simulation that this algorithm and this model have strong feasibility, efficiency and stability.