Grid-supporting power converters are gradually replacing the traditional synchronous machine power supply. It is an inevitable trend as new power systems are built. There is not a consensus on the purpose and physical meaning of dual-loop control for grid-supporting power converters. First, in order to elaborate the essence of inner current loop in dual-loop control, this paper analyzes damping characteristics of the current inner loop from the discrete root locus curve and addresses physical meaning from the perspective of virtual impedance. Secondly, to obtain the parameters of dual-loop controller quickly and accurately, avoiding iterations, and considering the delay caused by the sampling, calculation and PWM modulation of the digital control system, the inner loop uses a proportional controller to adjust the damping characteristics of the current inner loop. The proportional coefficient of the current controller is obtained from the optimal damping. The coefficients of the voltage controller are determined by the requirements of the closed-loop output gain at the filter resonance frequency and compensating offset angles of harmonics. Finally, simulations and experimental results have verified the effectiveness of theoretical analysis. This work is supported by the Science and Technology Project of the Headquarters of State Grid Corporation of China (No. SGAH0000KJJS2000680).