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.