Resonance frequency for the LCL-type grid-tied inverter in weak grid condition is proved essential because of its influence on stability of the system and quality of the grid-side current. Based on stability region criterion, to select the resonance frequency properly, this paper mainly takes two considerations into account. One is the grid impedance variation; the LCL resonance frequency is designed based on the geometric mean method to adapt to this variation in a wide range. Another one is the background harmonics; the significant impact on grid-injected current due to the interaction of multi-frequency is analyzed to determine an optimum value of the LCL resonance frequency. Combined with the converter-side current ripple formula, an effective and robust method to derive LCL filter parameters without external damping is established. MATLAB & Simulink are used to verify the effectiveness of the proposed method.