Dear @luo.luo,
please keep your FLUKA and Flair installation always up to date, as support only provided for the latest releases.
To create the trajectory plot, you need to add an USERDUMP card to your simulation, run it with a low number of primaries.
When it finished, you need to create a new layer on the Geometry window, with the Userdump option. See:
More explanation in the Flair manual: F4.13} Layers.
Finally here is an example Flair project file: track.flair (1.6 KB)
Cheers,
David