Mobile Applications has become an important part of people’s life. ABI institute once predicted that there get to have 8 hundred million Android phone and 3 hundred million iPhone in the world at the end of 2013. The tremendous business market behinds the little smart phone. Therefore, whether mobile app wins customer’s heart is the most important thing to designer. But there’s no particular research to tell how to design mobile applications to target its best interest. If the mobile application didn’t win people’s heart, it won’t make any profit and publicity. This research focuses on the key success factors during the process of researching and Designing a mobile application. Using literature analysis, the research combined Information System Success Model and virus marketing method and social factors of theory of planned behavior and interview survey with professional to build up the KDF table. And by applying the fuzzy-Delphi method, we confirmed all the dimensions and indexes are agreed with professionals. Analyze by DANP to find out the weights of each dimension and factor. According the result, we have found out that the most important dimension is the dimension named ‘APP service quality’. Secondly, the most important factor is User promoting. This research suggests that mobile app designer not only must focus on the app itself but also the social system, the better the product is, it needs more promotion and social effect, therefore more people would attracted by it. The social networks factor is a important promotion tool for app. These results would help avoid extra time and costs.