In paralleled inverter systems, the non-passive region where the synthesized admittance exceeds the ±90° passivity boundary is one of the key causes of instability in grid-connected operation under weak grid conditions. To address this issue, a heterogeneous inverter parameter configuration method based on admittance optimization is proposed. By appropriately configuring the sampling frequency and LCL filter parameters of newly added inverters, the non-passive regions of the synthesized admittance can be eliminated, thereby enhancing the passivity-based stability of the system without modifying the control structure and parameters of existing inverters. First, based on the Norton equivalent model of parallel inverter systems, stability criteria for the grid-connected current and the coupling current between inverters are established. The grid-connected phase margin and coupling phase margin are defined to reveal the relationship between the violation of the ±90° passivity boundary of synthesized admittance and harmonic resonances among multiple inverters. Then, by combining the resonant and anti-resonant characteristics of the LCL filter with the admittance synthesis rule, coordinated design guidelines for the sampling frequency and filter parameters of heterogeneous inverters are derived. A feasible sampling frequency range for newly added inverters is obtained to ensure that the synthesized admittance satisfies passivity requirements over the entire frequency range. Finally, the effectiveness of the proposed method is verified on an experimental platform consisting of two three-phase grid-connected inverters, each rated at 13.2 kW.