I am going to have a hair transplant. According to instruction, I need to stop taking Aspirin before 7 days operation.

It is best to contact your doctor about pre operative care and instructions since each doctor has a different regimen.It would be in your best interest to follow your doctor's recommendations.Aspirin causes increased bleeding.

OTC aspirin thins the blood causing more bleeding during a surgery.  This can cause many negative things in rearguards to your transplant, always follow your Dr.'s instructions.  It can also cause the surgery to take longer if you have more bleeding and grafts to "pop".  

Aspirin affects the function of platelets which are important components of the blood's ability to clot. Doses over 75mg daily are likely to cause increased bleeding during surgery.If aspirin, heparin, warfarin or other forms of blood thinners have been prescribed by a general practitioner, cardiologist or other doctor then you should check if it is OK to stop taking them prior to doing so. They are likely to have been prescribed for a life threatening condition and stopping them to have a cosmetic procedure needs to be carefully considered. 

Aspirin use will predispose you to bleeding during and after the surgery and ideally should be stopped a week prior to surgery if possible.  If you are taking the aspirin under a doctor's care for something like stroke or heart attack prevention, or if you have a stent in place, you should check with the prescribing doctor if it is safe to hold it.  If not, discuss this with your hair transplant surgeon in advance.  If you take it for pain, you can use something like Tylenol (acetaminophen).  Things like Motrin (ibuprofen) can also cause bleeding and are best avoided prior to surgery.If you can't/don't stop aspirin, be sure to notify your surgeon prior to the day of surgery.

If you have a reason like a heart valve or some other similar condition that requires Aspirin, I would do the surgery with you on the drug.  But if you just take it prophalactically, the follow the instructions. Speak with your doctor