This paper attempts to take user behavior as the research goal of service design, and introduces the Octalysis (octagonal behavior analysis) method into service design to provide innovative ideas and guiding strategies for service design and user research. The research is based on Octalysis, user experience and service design theory, combined with literature research to construct an SD-octagonal model suitable for service design. Therefore, a "Two-dimension & Five-step" design iterative strategy based on Octalysis is proposed. Taking fitness institutions as an example, through the macro and micro perspectives, two-dimensional constraints are imposed on the static service system and dynamic user behavior. At the same time, combined with the five-step iterative strategy centered on the SD-octagonal model, the user experience process is expanded to analyse according to user type and experience time, so as to complete a three-dimensional, multi-level user behavior analysis. Integrate game thinking into service design, assist designers to conduct user research more accurately and comprehensively, so as to find design opportunities with different feasibility and provide effective information for the service strategy.