Objective This study analyzes the effect of the shrinkage of thermoplastic mask on patient positioning. Methods Design of the two test. Test 1:thermoplastic film shrinkage test. Get some thermoplastic film by the size of 10 cm×5 cm, extrude it at a certain rate after heated. Measure the length of thermoplastic film on different time, and calculate the contraction. Test 2:phantom test. Take advantage of head and neck phantom, and simulate the procedure that from making mask for patients to radiation therapy. Measure the off set of isocenter which caused by the contraction of thermoplastic mask. Results The largest shrinkage of thermoplastic had happened in 20 minutes. Different tensile ratio had little effect on the shrinkage. The offset of isocenter which caused by the shrinkage of thermoplastic mask were:LR ( -0?? 1± 0?? 3) mm,SI (-0?? 2±0?? 2) mm, AP (0.6±0?? 4) mm,respectively. There was little change in the course of six weeks ( P= 0.185?0?? 961). Conclusions The cooling time should be more than 20 minutes, when making a mask for the patient. The setup errors which caused by the shrinkage of thermoplastic mask is at an acceptable level on this premise.