The investigators,according to the eligibility criteria,plane to choose 80 elderly anticipates who will undergo selective hip replacement surgery.This participants will be randomly divided into 2 groups:light anesthesia depth group(L Group) and deep anesthesia depth group (D Group).Before anesthesia induction,each participant will be given lumbar plexus and sciatic nerve block in order to control pain of interoperative and postoperative.During operation period,the L Group will be kept in light anesthesia depth(BIS:50-60),while the D group will be kept in deep anesthesia depth(BIS:35-45)--The BIS values in the range of 30-60 have been recommended in general anesthesia.After operation,a PCIA(patient controlled intravenous analgesia) analgesia pump will be given to each participant to control postoperative pain. The investigator will evaluate the mental state of participant with MMSE(mini-mental state examination) before surgery and review each participant with a CAM-CR for 3 days after surgery to assess the situation of postoperation delirium.intraoperative awareness,postoperative pain will also be investigated in postoperative days 1, 2, 3.

The investigators,according to the eligibility criteria,plane to choose 80 elderly participants who will undergo selective hip replacement surgery.This participants will be randomly divided into 2 groups:light anesthesia depth group(BIS:50-60) and deep anesthesia depth group (BIS:35-45)--The BIS values in the range of 30-60 have been recommended in general anesthesia.Before anesthesia induction,each participant will be given lumbar plexus and sciatic nerve block in order to control pain of intraoperative and postoperative.The investigator use propofol 4ug\kg、and sufentanil 0.2ug\kg during anesthesia induction period,after that,a Laryngeal mask will be placed into patient's mouth.Anesthesia is maintained with remifentanil TCI(target-controlled infusion) target-controlled infusion(concentration:2ng/ml) and propofol BIS closed-loop target-controlled infusion.During the operation period ,the investigator keep the L Group's BIS range from 50 to 60.While the D Group's BIS range from 35 to 45.All participants receive postoperative analgesia.After operation,all participants will be send to PACU(post-anaesthesia care unit ) to wake up and send back to ward,And a PCIA(patient controlled intravenous analgesia) analgesia pump will be given to each participants to control postoperative pain.The investigators will use MMSE(mini-mental state examination) to evaluate the participant's mental state before surgery.For 3 days after operation,The investigator will review each participant with a CAM-CR to assess the situation of postoperation delirium.The investigator will see if there is intraoperative awareness by asking questions.Postoperative pain(with VAS) will also be investigated in postoperative days 1, 2, 3.

Inclusion Criteria:

1. participant who is selected to undergo hip replacement surgery.
2. participant who is older than 60 years old.
3. participant whose ASA（The American Society of Anesthesiologists） is II-III.
4. participant whose BMI is 18-25.
5. participant who is able to understand and complete The Confusion Assessment Method(CAM-CR).
6. participant who is able to finish Visual Analogue Scale(VAS).
7. participant who is conscious and willing to take part in this research.
8. participant and his/her relatives are willing to take part in this research.

Exclusion Criteria:

1. participant who has serious liver or kidney disease.
2. participant who suffer from mental or neurologic disease,such as Alzheimer disease.
3. participant who has undergone cardiovascular surgery or neurosurgery.
4. participant who has have sedation or antidepression medicine for a long time.
5. participant who has something wrong with his/her vision or hearing.
6. participant who has blood coagulation dysfunction.
7. participant who is an illiteracy.