In HCI, mental models facilitate user-system interaction and improve usability. However, constructing a comprehensive mental model in enterprise systems such as cloud platforms presents challenges due to the varied user needs and intricate product logic. In this paper, we propose a three-dimensional framework (User, System, Time) to tackle this issue. The USER dimension outlines biological, social, consumption, and usage characteristics. The SYSTEM dimension breaks down products into entities, attributes, and functionalities. The TIME dimension outlines the user journey stages. By combining variables across these dimensions, we can generate targeted research units that collectively form a comprehensive mental model for complex products. Our method also includes a corresponding workflow and provides a toolkit for essential processes. Tested on 10 Alibaba Cloud products, we developed complete mental models in four months through more than 20 studies. The results indicate that it effectively guides user insight exploration, identifies UX improvements, and offers actionable methods for constructing mental models in enterprise products. The framework is adaptable to other complex systems.