I drew a smooth line circle with Inventor(AutoDesk’s soft), converted to stl file and imported to Ideamaker.
But when I done slice project, this circle was drawn as “An Angular Line Circle”.
I want to make a smooth line circle.
What should I do for drawing a smooth line?

That’s actually the nature of most 3d model file formats, stls along with some others make everything in the model out of triangles so an stl is always going to look like that though you can raise the resolution of the export more so there’s more triangles, closer to the shape you want. Some software will take stls and I think use interpolation, I could be wrong, but basically it’s to smooth things out hopefully closer to what it was originally. But usually you won’t notice it in the print unless the resolution is really low. Tldr: that’s normal, just turn up the export resolution

Click on save as CAD format and choose STL but before clicking on save, click on options, This opens another window, make sure the highest quality check box is ticked click on ok then save the STL file. When you open the stl file in your slicer it should be round.

It could be a number of things but when full parametric models are imported to stl, they’re converted into a mesh or polygons. As mentioned in the comments use the highest settings when converting into stl format. It could also be the software you’re using. Open the same stl file in another software (such as Mesh Mixer) and see if its rendering the stl object as a smooth surface.