Salter's Duck can convert wave energy to mechanical energy with high efficiency up to 90%. The limitations and challenges in the usage of Salter's Duck for energy production include preventing hydraulic system from the risk of leaking hydraulic oil and removing complex huge fixed bracket. This paper presents the conceptual design and an experimental investigation of a mechanical power-take-off (PTO) system for the floating Salter's Duck. The PTO system is fully installed inside of the Salter's Duck. It consists of double counter-rotating flywheels converting the bi-directional rotation of the wave energy converter into a stable unidirectional rotation that drives a rotary generator to produce electricity. The bi-to-unidirectional function is achieved through inertial wheels, thus the novel PTO system is free of large supporting structures and hydraulic cylinders. In this paper, a detailed conceptual PTO design is proposed firstly. Further, the influence of the swing amplitude and rotation period of the wave energy converter on the power extraction efficiency is investigated. Then, the influence of the electrical load on the power-extraction efficiency is researched. In the end, the effect of the speed increasing ratio on the mechanical efficiency is examined. The experimental results show that the new type of generator mechanism can produce electricity stably.