Aquatic animal movement results from a complex balance between muscular actuation, swimmer's inertia, damping, and stiffness; as well as, the effects from the fluid environment. Most aquatic animals utilize undulatory propulsion methods during swimming. Propulsion mode transition involves a variation of these parameters, and to better investigate the variation of these parameters during propulsion mode switching, and provide guidance for swimming robot design, we studied propulsion mechanism of undulation locomotion by combining biological investigation, mathematical simulation and experimental validation. A modular robot platform, with assembling function, was built based on the obtained biological features to realize the corresponding propulsion methods. Then a modular dynamic modeling method was proposed to simulate robot locomotion using a CPG based algorithm and a PD control method, further revealing the underlying mechanism for undulatory locomotion. Finally, experiments were conducted using the robotic platform to validate the found conclusions as well as enhance the propulsion mechanism of undulatory motion, providing a generic guidance for swimming robot design.