Abstract: The structural design of light pen for a space coordinate measuring system without guideway is expounded in detail. 3/4G redundancy technique is applied in the design of the light pen system for the realization of self-calibration and self reparation when one light source is deficit for any reason. A self-calibration algorithm is set up through simulation calculations. The error of light pen system is less than 0.1 mm when the coordinate error of point source is (±0.1 mm.) An original way of frequency-recognition that applies light source of different modulation frequencies on the light pen is put forward to identify multi point sources.