This paper presents the behavior design and multimodal locomotion control of a biomimetic amphibious robot based on a bio-inspired CPG (central pattern generator). A set of four key parameters are introduced serving as external stimuli to shape the CPG rhythmic activities where necessary speed and orientation modulation as well as 3-D locomotion can be obtained. In terms of the built parameter set, a library of movement primitives based on finite state machine is established to facilitate rapid and smooth gait transitions. To enhance adaptive behaviors, well-integrated sensory feedback by means of two liquid-level detectors enables the gait transition between ground and water autonomously. Simulations and experiments are also conducted to demonstrate the feasibility of a behavior-based control architecture governed by CPGs.