Product Service System (PSS) can provide customers integrated solutions including products and services. It can create new developing opportunities for manufacturing engineering through the effective integration of services and products. The traditional design theories and methods cannot meet the new design requirements when considering PSS's fusion of design objects and complexity of design process. Reliable design of PSS is required to make full use of quantitative and intelligent methods. From the three aspects of unified modeling, design decision making and design optimization, the existing problems and future research directions are proposed based on summarizing theoretical research and practical application of domestic and international situation.