Purpose <p>The bidirectional gear-driven friction nanogenerator can convert mechanical energy into electrical energy through transmission system and energy acquisition technology, which is of great significance in the development of miniaturization, intelligence and greening. Therefore, this study aims to explore the relationship between the dynamic characteristics of mechanical transmission system and the mechanical energy conversion efficiency of bi-directional gear-driven friction nanogenerator system.</p> Methods <p>Considering the time-varying mesh stiffness, time-varying support stiffness, transmission error, tooth side clearance and bearing clearance, the nonlinear dynamic model of the mechanical transmission system of the bidirectional gear-driven friction nanogenerator is established. The Runge Kutta method was used to solve the vibration differential equation of a mechanical transmission system, and the influence of external load excitation frequency on the dynamic characteristics of the system was analyzed. The influence mechanism of external load excitation frequency and average mesh stiffness on the mechanical energy harvesting of the system was analyzed by combining the friction nanogenerator (TENG) energy harvesting technology.</p> Results <p>There are abundant nonlinear phenomena in the mechanical transmission system of bi-directional rack-driven friction nanogenerator. With the increase of the external load excitation frequency and the average meshing stiffness, the vibration characteristics of the mechanical transmission system will experience three different motion states, the power generation of the system will increase, but the mechanical energy conversion efficiency of the system will decrease.</p> Conclusions <p>The results show that the mechanical energy conversion efficiency can be improved and the power generation of TENG can be increased by reasonably selecting the external load excitation frequency and meshing stiffness and avoiding the unstable region.</p>