User stories are widely used in agile development projects. Every user story tells what the user/customer wants the system to do. However, a user story can only contain a small piece of the requirements but not the whole business logic. That means that when the customers submit user stories, the developers need to combine them together according to the relationships among them for producing or updating the system requirements. That is very tedious, time-consuming, and error-prone. This study proposes a human-machine collaborative approach to support the user story understanding and system functional requirements generation. This approach proposes to include the scenarios as the fine-grained representation of user stories and presents a feature-scenario model to capture the elements of user stories in three dimensions, i.e., the story description, the function attributes, and the scenarios. It designs a three-step algorithm to accept the submission of user stories, extract the features of each user story, and construct its functional scenario. As there are relationships among different user stories, it defines three types of correlative relations among them based on the functional scenarios. With the help of the customers, it supports the measurement and the identification of these relations and then constructs the system's view of the functional requirements. It is also applicable when obtaining new user stories for tolerating the requirements evolution. A case study shows the feasibility of this approach.