Objective To classify and analyze irrational drug use in pharmacy intravenous admixture services (PIVAS) of a tertiary hospital, and to provide a reference for improving the rational use of drugs and ensuring the drug use safety of patients.Methods A retrospective survey method was used to sort out the long-term medical orders of a hospital from January 2018 to August 2021, classify irrational medical orders, and analyze the situation of irrational drug use.Results Among the 490 269 long-term medical orders, there were 2 739 irrational medication orders, of which the number of irrational long-term medical orders in 2019 was the largest. Among the types of long-term medical orders for irrational drug use, the top 3 errors in issuing-medical- orders were that drugs and solvents were not grouped together (8.21%), the number of changes made by physicians (1.79%), and the repeated issuing of medical orders (0.33%). The top three errors without prescriptions of doctors were irrational selection of solvent types (32.06%), irrational selection of solvent dosage (19.46%) and inappropriate drug dose (16.43%).Conclusion Prescription pharmacists use professional pharmaceutical knowledge, relevant laws and regulations and medication system software to carefully review the contents of prescriptions or medical orders, timely intervene and find unreasonable drug use, so as to reduce the occurrence of adverse drug reactions and improve the safety and effectiveness of drugs.