Satisfying quality requirements for service systems is quite crucial and challenging. However, there is a gap between quality requirements analysis and quality requirements design in service systems. In order to bridge this gap, we provide a systematic approach — ProQRASS — to model and analyze quality requirements of services based on business processes, which are frequently used to model services. ProQRASS consists of five steps: (1) constructing business process models; (2) associating quality requirements with functional requirements of services in business process models; (3) identifying potential conflicts and cooperation among quality requirements; (4) filtering out false conflicts and cooperation; (5) resolving conflicts among quality requirements. We illustrate ProQRASS through an equity trading service system. We also evaluate its capability through the comparison with other approaches and conduct a usability investigation involving industrial experts. The result shows that ProQRASS is effective and useful.