Abstract: Using 3 CCDs to detect the position changes of 3 collimated beams, a six-degree-of-freedom measurement method was presented. The measurement system consisted of two parts. A mirror and two CCDs that were connected together by a link rod form the moving part. It was fixed to and moving along with the measured object. A light source, an optical switch and three optical collimators of the fixed part emitted 3 collimated beams. Two collimated beams projected on two CCDs respectively. The third one was reflected back by the mirror and captured by the third CCD. Six-degree-of-freedom motion of the measured object led to the synchronous change of the moving part, resulting in position changes of 3 light spots on CCDs. Then with a set of algorithms, the six-degree-of-freedom motion of the measured object can be calculated based on these position changes. A four-degree-of-freedom measurement system, which can manifest the characteristics of six-degree-of-freedom measurement, was built and experimental tests were carried out. Within the corresponding measurement range, compared with the results of the dual-frequency laser interferometer and photoelectric auto-collimator system, the maximum deviations of yaw and roll in this system are 1.27 and 1.03, and that of two translational movements are less than 1 m.