Radiotherapy causes atrophy and fibrosis of the masticatory muscles, resulting in patients experiencing difficulty in opening their mouths. Difficulty in opening the mouth mainly lies in prevention, patients during radiotherapy should carry out mouth opening exercises, massage the temporomandibular joints, or take a triangular tongue depressor to gradually make the mouth open as wide as possible. Patients can also chew more gum. Exercises to open the mouth so that it opens as wide as possible should be practiced 3-5 times a day for 15-20 minutes each time. Patients can also do rotational neck exercises at the same time to prevent atrophy and fibrosis of the neck muscles. The focus is on preventing difficulty opening the mouth. If muscle fibrosis is already present, treatment is very difficult.