The invention discloses a method for measuring fitness of a thermoplastic film with the surface of a human body under infrared guide. The deviation, caused by the unconscious displacement of the human body, of the thermoplastic film relative to the human body in a treatment process causes the position of a focus center relative to the thermoplastic film changes, which will causes X-rays not to irradiate a tumor target region or not to irradiate the tumor target region at an expected high dosage, so the treatment effect is affected heavily. The method can be used for detecting a space position deviation of the thermoplastic film and the surface of the human body in a radiotherapy process, and achieves the film-surface calibration according to the deviation.