Objective To investigate the cause of abnormality increase of international normalized ratio (INR) (INR>4.5) in the patients who oral application Warfarin. Methods 148 patients were taken who had cardiac valve replacement operation and oral application warfarin post-discharge as subjects. Their INR abnormality increased when taking reexamination. Through the research of the drugs, the dose (including both Chinese crude drug and western medicine), the nourishments and the foods that the patients took, we find out factors that resulted in INR abnormality increase, while to investigate the quality control of laboratory department and analyze the process before and after PT test, to find out factors that affect the test. Finally, to analyze the causes of INR abnormality increase in patients took warfarin after cardiac valve replacement operation. Results The main reason for the abnormal increase in INR in patients with oral warfarin was the excessive dose, accounting for 47.29% of all surveys, of which 39.18% were excessively prescribed, and the proportion of patients who forgot to repeat or did not understand the doctor's order was 8.10%; The proportion of drug interaction was 43.25%, of which 33.11% were affected by western medicine, 10.14% were affected by Chinese medicine, 9.47% were affected by external medicine, and 4.06% were affected by diet; The proportion of treatment effects was 5.41%. Conclusion The main reason for the abnormal increase in INR in patients with oral warfarin is the excessive use of drugs and the interaction of other drugs, as well as the effects of diet and physical therapy.