The grid-forming (GFM) inverter, which has the function of supporting grid voltage/frequency, has drawn much attention. However, like the grid-following (GFL) inverter, the inverter-grid interaction also poses instability risk to the GFM inverter. To address the stability issue, the passivity-based approach has been widely applied to GFL inverters, and is proved to be effective and promising. In this paper, we applied the passivity-based approach to the GFM inverter and focus on the impact of power control parameters on the system passivity. The findings will contribute to the controller parameter optimization of the GFM inverter.