The efficiency and quality of current collaborative discussion for problem-solving can hardly satisfy the practical demands. To solve this problem, a formal model of collaborative discussion for problem-solving is established. First, to express thinking transformation in the process of collaborative discussion, the model of PTMM (Participant-driven two-directional mind map) is proposed, and then the formal definitions and operations for PTMM are provided. Second, the solutionspace theory of PTMM is investigated. To let the solution be clearer, the concept and generation process of STMM (Solution-driven two-directional mind map) are given, which are derived from the PTMM. Third, inspired by the rhombus thought in extension theory, the RSM (Rhombus sequence model) based on PTMMand STMMis obtained and shown dynamically by visual means. Lastly, a practical example of collaborative discussion is shown, demonstrating that the RSM is able to effectively enlighten the thinking and promote the efficiency together with quality of collaborative discussion.