A system for conversion of mechanical energy into electrical energy by electrostatic conversion that includes at least one fixed electrode (2) and at least one mobile electrode (4) that can be vibrated by mechanical energy, facing the fixed electrode, said fixed (2) and mobile (4) electrodes being separated at rest by an air gap distance (Δ). The system includes means (12) of modifying the air gap distance as a function of the variation of the vibration amplitude of the mobile electrode (4). An energy conversion method using such a system is also described.