Looking for suggestion, which is the best workstation laptop. which makes easy for converting 400MB STL file to STP file in catia V5 cad software.

"Best" is a term which only you can define.

If I were looking to buy a laptop workstation, I'd:
- Determine how much I can spend
- determine which components will give me the "best" return for my money. i.e. do I want a faster CPU, or more cores? More RAM?, Faster SSD disks, or more capacity with rotating platters? A newer/more expensive video card?... etc.
- I'd then go to eBay and search for a Dell Precision M6600, M6800, or 7720

I choose the Dell models because I have used them for years. You may also look for HP, Lenovo, or other brands if you have a preference.
I prefer to buy 2 year old used laptops which are coming off of a business lease since I can save thousands of dollars versus buying a band new laptop.

How do you plan to convert an STL file to a STP file?
I've never used Catia, does it have a tool which wraps surfaces around mesh models?
If the goal is a parametric model with wonderful topology, you'll mostly end up remaking the entire model feature by feature.

Most CAD programs do not require very powerful hardware unless you are doing very complex things like analysis, rendering, or working with tens of thousands of parts in an assembly.

A refurbished HP 2760P 12.1" Laptop, Touchscreen, Windows 10 Pro, Intel Core i7-2620M Processor, 8GB RAM, 120GB Solid State Drive, could be a good choice.