Under the guidance of core competencies, the reform of primary school mathematics teaching urgently needs to go beyond the superficial teaching of knowledge and skills, and construct a new teaching paradigm aimed at promoting students’ conceptual understanding and endogenous literacy. Addressing the long-standing problems in the field of graphics and geometry, such as fragmented knowledge, superficial thinking, and vague literacy goals, this study takes Erickson’s “concept-based” curriculum and teaching theory as the meta-framework, integrates domestic research on big idea teaching, and constructs a three-dimensional collaborative large-unit teaching model of “Knowing—Doing—Understanding” (KUC). Taking the unit “Parallelograms and Trapezoids” in primary school mathematics (People’s Education Press edition) as an empirical carrier, the paper systematically elaborates the complete design path from “extracting subject big ideas” and “establishing conceptual perspectives” to “designing hierarchical guiding questions,” “constructing performance evaluation” and “sequencing iterative learning activities,” and develops an anchoring performance task of “campus transformation designer”. Practice shows that this model can effectively drive students’ thinking to leap from “factual memory” to “concept construction,” providing a theoretical reference and practical model for the transformation of competency-based classrooms.