Insufficient inertia has retrained the progress toward a one hundred percent inverter-based power generation. To solve this issue, a promising way is to exploit synthetic inertia based on the power electronic inverters. Taking advantage of energy stored in the DC side capacitor, this paper proposes a synthetic inertia control scheme for inverters without a DC side battery. The basic mechanism of the synthetic inertia control is derived by comparing the working principle between a synchronous generator and a three-phase inverter. Thereafter, the control algorithm of the inverter is designed by three parts, including synthetic inertia control loop, reactive power control loop and line oscillation damping control loop. Finally, simulation results based on a simplified microgrid are presented to verify the self-grid synchronization and grid-supportive characteristics of the inverter.