I am a Tunisian PhD student, at my last year of PhD, I found a lab from UK and I want to do a visiting research PhD with them (during 7 months), PS: I will finance myself. I don't have a big background about the visa. Thus, I have a question Which type of visa I should applied for ?
Because you are registered for your PhD program abroad, you likely can likely visit UK research institutions on a Standard Visitor Visa. You may need to complete ATAS before applying for a visa. Depending on your exact situation, a work visa may be more appropriate.
Applying for a UK visa is an expensive and long process. It may be wise to ask your host organisation for help before you apply.