Objective To analyze the current situation and causes of protocol deviations in anti-tumor drug clinical trials, in order to reduce the occurrence of protocol deviations.Methods Retrospective analysis and root cause analysis (RCA) were used to collect protocol deviation reports from 99 clinical trials of anti-tumor drugs from May 2023 to June 2024. The responsible subjects, types and specific reasons of protocol deviations were retrospectively analyzed, so as to reduce the incidence of protocol deviations from the root problem.Results 99 clinical trials of anti-tumor drugs had a total of 642 cases of protocol deviations, and the subjects and investigators were mainly responsible for the protocol deviations. The main types of protocol deviations included exceeding the allowed time window of protocols, missed/no visits or lacking of examinations/other procedures in the visit, drug-related deviation, biological samples-related deviation, adverse event (AE)-related deviation, system entry deviation, etc. The main reasons for the protocol deviations included human factors such as subjects' own factors, the investigators' familiarity and rigor with the anti-tumor drug trial protocol, and clinical research coordinator's cooperation and quality control of the trial, as well as the influence of material, environment, process, sponsor, and holidays factors in the drug trial. The occurrence rate of protocol deviations in domestic projects was higher than that of foreign ones.Conclusion Protocol deviations still widespread in clinical anti-tumor drug trials. Subjects and researchers are the main responsible parties for protocol deviations. So it is urgent to strengthen the quality control of drug clinical trials. By making deep analysis of causes and providing guidance for the effective improvement measures, it is expected to reduce the protocol deviations and improve the quality of clinical trials of anti-tumor drugs.