The simple pendulum is a classic model that serves as an important bridge between secondary-school and university physics. Drawing on the teaching practice of the Lujiaxi Class at Northeast Normal University, this paper examines the breakdown of the small-angle approximation for the pendulum period and guides students to derive the dynamical equation of the simple pendulum from three perspectives: Newton’s second law, the law of rotational motion, and the conservation of mechanical energy. Through numerical simulation, Taylor-expansion-based corrections, and elliptic-integral analysis, students progressively gain a deeper understanding of the relationship between approximate and exact solutions. The teaching practice demonstrates that this problem-driven, inquiry-based approach, built around a classical physical model, facilitates the integration of secondary-school and university knowledge, fosters students’ innovative thinking and research literacy, and offers a practical paradigm for cultivating top-notch students in the context of a secondary-university continuum.