Grid-connected converters have been widely used in distributed generations. However, the resonance brought by converters in weak grid has been a great threat to system stability. Impedance-based method provides a powerful tool for stability analysis. Active damping is also widely applied to improve system stability. This paper derives the impedance model of grid-connected converter system and analyzes the harmonic resonance mechanism. The effects of passive damping and active damping are also compared, especially their influence on non-passive region. Impact of capacitor current feedback gain on the output admittance is also analyzed. High-pass filter is used to compensate the digital control delay and reduce the non-passive region of output admittance. An improved active damping is proposed to eliminate second harmonic introduced by active damping. The performance of proposed method is verified in Matlab/Simulink simulations. The simulation result shows the effectiveness of proposed method.