How can i make a smooth deformation line using plaxis 2d
I think this is how your data looks like and you cannot make it any smoother. But if you want to modify furher, you may re-draw it in Excel by taking the data using CTRL+T (Table view.)
The curves program will generate proper connections with the individual points. Note that cross-sections will use interpolated and extrapolated data to generate this cross-section curve for. Also, if you have a discontinuity in your model, e.g. a soil layer change, structural elements, then PLAXIS will not assume that the line is continuous, and so you could see a small discontinuity in the line. This can also be observed in the normal cross-section plot (zoom in around this location).If the mesh is dense enough, usually you do not see this.
If you want to smoothen this data, you could, as Mr. Demir mentioned, export your data using the table function to copy it and then smoothen it in a spreadsheet program like Microsoft Excel.