[Purpose/significance] Taking English paid APP Fluent Reading as an example, this study aims to explore the related factors affecting users' behavior of paying APP for knowledge and its mechanism, and provide suggestions for the sustainable development of knowledge payment platform. [Method/process] Based on UTAUT model and related literature, the hypothesis is put forward; using the interview method and questionnaire survey method ;using SPSS22.0 to analyze quantitatively the collected data and modificate the model. [Results/conclusion] The study found that the degree of recognition of expectation and social impact had the most significant effect on the user's willingness to use payment APP. Therefore, it is suggested that the knowledge payment platform should create a good media image outside, and improve the information quality, so as to improve the user's expected confirmation value; at the same time, explore the social resources of the user network and use the social impact to stimulate the user's willingness to use to promote their usage behavior. In addition, the research also proves that the introduction of the degree of expectation confirmation achieves the development of the innovation of UTAUT model.