Continuance intention is an intuitive projection of a user's intention to continue a certain behavior, and it is also an intuitive manifestation of APP user viscosity. The work aims to study the influencing factors of fitness APP user viscosity to improve user experience in fitness. On the basis of the continuance use model of the information system, the literature research method was used to introduce variables such as self-efficacy and social influence in the social cognitive theory to establish a hypothetical model of APP viscosity influencing factors. Data on 233 users were collected for further reliability and validity analysis. The structural equation model was used to measure the correlation degree of each variable. From a dual-theory perspective, each variable was used as a factor in the design of the fitness APP system. The factors directly influencing APP user viscosity were:self-efficacy, perceived usefulness and satisfaction. Indirect factors include social influence and convenience. According to the orientation of various factors, a holistic design strategy aiming at enhancing the user viscosity of fitness APPs was proposed:the design should be guided by the six principles of easy availability, competence, feedback, unexpectedness, and sociality.