An FFT can be run simply by setting up a .FOUR directive in the simulation schematic.

Please note that EasyEDA does not currently support:

1) directly plotting the results of an FFT in WaveForm. The results are listed in the Simulation Report and can be plotted in any spreadsheet or other data plotting software.

2) the FFT functions available in the native LTspice plot GUI.

For advanced FFT analysis, it is recommended to download the LTspice netlist from EasyEDA and to then run that in a local installation of LTspice.