Vibration energy harvesters with bistable characteristics, which can convert mechanical energy to electric energy, typically are cantilever beams with magnetic repulsion. In group to enhance its low-frequency performance, a seesaw-type approach has been proposed, which can make the structure overcome the potential barrier more easily. In this paper, we establish electromechanical coupling equations of the whole system, and prove that internal beam delivers mechanical energy to primary beam based on time-domain analysis. Meanwhile, frequency-domain analysis and solutions are conducted to investigate dynamical and electrical behaviours of the system based on the adjustment of different parameters. We find both mechanical and electrical responses are enhanced gradually with the decrease of relative damping coefficient within the bounds of the discussion. And maximum response amplitude can be increased by a factor of 11.3 just by adjusting the position of the internal beam. Furthermore, the length of internal beam can affect responses of the system visibly and regularly, and maximum response amplitude remains unchanged when changing the length of internal beam from 80 mm to 100 mm. All of these suggest that the mechanisms of seesaw-type energy harvester will provide guidance to design more appropriate structure depending on different applications.