Through data collection methods such as on-site observation, experience or interview, qualitative research can deeply understand the attitudes and opinions of doctors and patients on a certain phenomenon, explore the obstacles in the process of medical service and preventive health care, and obtain patients' treatment expectations and outcome evaluation. There are more and more applications of qualitative research in medical field, but researchers still face many methodological problems, one of which is how to determine and describe the sample size of qualitative research.