Pain is one of the most common and fear symptoms for cancer patients, which seriously affects the quality of life in cancer patients. At present, oral opioid is the most common route to administrate cancer pain. However, the patients do not satisfy the pain administration with oral opioid after successful titration in many cases, especially the cases with severe cancer pain. Patient controlled analgesia (PCA) with hydromorphone can take analgesic effect rapidly. The aim of this trial is to compare the maintenance with hydromorphone PCA intravenously or switch to Sustained-Release Morphine orally after successful titraton with hydromorphone PCA intravenously in severe cancer pain.

Inclusion Criteria:

1. The patients were 18-80 years old and diagnosed as malignant tumor by pathology;
2. Patients with cancer pain is NRS pain score ≥ 7 during previous 24 hours;
3. Patients who will not be treated with radiotherapy within 7 days prior to randomization and during study ;
4. Patients who need chemotherapy, long term administration of hormone, targeted therapy, or bisphosphonates therapy should undergo a stable anti- tumor therapy prior to randomization ;
5. Patients or his/her caregivers who are able to fill out the questionnaire forms ;
6. Ability to correctly understand and cooperate with medication guidance of doctors and nurses ;
7. Without psychiatric problems;
8. ECOG performance status ≤3;
9. Not participated in another drug clinical trial within one month before inclusion（including hydromorphone）;
10. The subjects voluntarily signed the informed consent.

Exclusion Criteria:

1. The pain is confirmed not due to cancer;
2. Patients with severe post-operative pain;
3. Patients with paralytic ileus;
4. Patients with brain metastasis;
5. Patients hypersensitive to opioids;
6. Patients with abnormal lab results that have obvious clinical significance, such as creatine ≥ 2 fold of upper limit of normal value, alanine aminotransferase (ALT) or aspartate aminotransferase (AST) ≥ 2.5 fold of upper limit of normal value, or liver function of Child C grade;
7. Patients who cannot take drugs orally;
8. Patients with an incoercible nausea or vomiting;
9. Those who have received monoamine oxidase inhibitor (MAOI) within two weeks before randomization;
10. Patients who are pregnant or in lactation, or who plan to be pregnant within one month after the trial;
11. Those with opioid addiction;
12. Alcoholic patients;
13. Those with cognitive dysfunction;
14. Those with severe depression;
15. Patients with other conditions or reasons causing the patients unable to complete the clinical trial.