Grid-forming converters can actively provide inertia and frequency support to the power grid. However, their external characteristics, similar to those of synchronous generators, may complicate the system oscillation characteristics, making oscillation source identification and controller parameter tuning more challenging. To evaluate system oscillation characteristics with grid-connected converters, this paper proposes a method for analyzing the oscillation characteristics and optimizing the parameters of grid-forming converters based on the oscillation energy method. First, a detailed model of the grid-forming converter and its oscillation energy model are constructed, and the accuracy of the energy model is verified. Based on this, the influence of controller parameters on oscillation energy is quantitatively analyzed using trajectory sensitivity techniques, allowing for the identification of the dominant control parameters that affect system oscillation state. Consequently, converter control parameters are optimized and tuned. Finally, a four-machine two-area system model with grid-forming converters is constructed in DIgSILENT/PowerFactory. The oscillation energy flow paths within the system are illustrated, verifying the accuracy of oscillation source identification and key parameters recognition, as well as the effectiveness of oscillation suppression after parameter tuning.