Background: Despite the great signs of progress in cancer pain management in China, the associated pain remains under-treated. Poor knowledge among the medical staff is an important factor contributing to the under-treatment of cancer pain. This study aimed to evaluate the knowledge, practices, and perceived barriers in cancer pain management among the medical staff at oncology units in China. Patients and Methods: A cross-sectional survey was conducted with the medical staff (including physicians, nurses, and pharmacists) at oncology units in tertiary hospitals of China between December 2020 and January 2021. The questionnaire assessed the knowledge, practices, and perceived barriers in cancer pain management. Results: A total of 1262 medical staff responded to the questionnaire; the response rate was 94.2%. Most participants had good knowledge of the three-step analgesic ladder of the World Health Organization (WHO) and the National Comprehensive Cancer Network (NCCN) guidelines for Adult Cancer Pain. Knowledge deficit was prominent in questions on opioid dose titration and rotation and adverse effects of opioids; the correct response rate was less than 40%. Training, work experience in oncology, and education level were significantly related to knowledge of cancer pain management (all P < 0.001). In clinical practice for cancer pain management, approximately 57.2% of medical staff were unfamiliar with opioid dose titration and rotation; only 14.4% treated cancer pain through multidisciplinary collaboration. Poor medication compliance, difficult individualized analgesia protocols, and insufficient multidisciplinary participation were the most frequently perceived barriers by the medical staff for pain management. Conclusion: These findings suggested a further need for integrating recent guidelines to strengthen continued training (especially among juniors and those with low education levels) and patient education to improve the knowledge and clinical practices of cancer pain management among the medical staff in China. Multi-disciplinary management is required for the effective treatment of cancer pain.