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Thread form showed distortion when measuring parallel screw thread parameters by using non-contact method. The traditional way had some defects, such as time consuming, skilled operator, high-precision equipment for adjustment and so on. Hence, a method through digital image calibration was presented. Based on the thread geometrical principle, the mathematical model was given out. With the above thread mathematic model, images obtained from camera were corrected and real thread form could be got. With the results of related experiments and simulation, the mathematic model was certified. This proposed method could improve accuracy and efficiency of thread measurement and detection.