In the engineering application and scientific research, it is usually needed to measure the instant rotational speed of rotating objects and the phase difference between two turning objects. But making an exact measurement of them is really not an easy job. The interface introduced in present paper enables this problem to be solved to a considerable degree. By means of it a highly exact measuring result of rotational speed and phase difference can be abtained in a signal circle after the starting command of the computer. In this article the principle of the interface, the maesuring error and the reliability of circuits are described in detail. Finally, a practical basic program for measurement is given.