Abstract: For the purpose of accurately predicting the welding distortion of steel structure,a three-dimensional stereo visual measuring system was developed to measure several marked points on the steel structures.The parameters for the welding distortion measuring system should be calibrated through two steps prior to measuring.First,the camera's inherent parameters were calibrated by the planar template method.Inherent parameters were composed of single camera's intrinsic parameters and position parameters between two cameras.Second,the position parameters between the camera coordinate system and designed coordinate system were estimated by unit quaternion algorithm.In order to verify the accuracy of calibration results,two camera's intrinsic parameters error was estimated by means of re-projection.Re-projection error was very small and the result satisfied the requirement of the system measuring error.The calibration results were verified with reconstruction experiment,whose average error was less than 1 mm with the maximum error less than 2.5 mm.