I'm from Pakistan and how can i reach to Changsha Medical University?

Well, upon arrival at Changsha, you can go to the local railway station to take bus no. 312 to West Bus Station, where you can transfer to bus no. 918 and get off the Medical University.
Or at the South Railway Station, you can take bus no. 63 or 348 to the West Bus Station and then take bus no. 918 to the university.