At present, there are two major methods to measure the cylindrical threads, one is the contact method by thread measuring machine and the other one is the three-wire method (and two-ball method) by length measuring machine. However, the measurement results by the two methods are different in some ways, which leads to controversy. This paper presents the structural principles of cylindrical screw thread measuring by thread measuring machine, constructs the geometric model of contour scanning, designs the algorithm of tip radius compensation, and analyzes the error factors of the testing method. The profile scanning method of thread comprehensive measuring machine proposed in this paper has the characteristics of high measuring accuracy, high speed, comprehensive parameters, automatic detection process and little human interference. It is the best method of thread comprehensive parameter measurement.