In order to meet the requirements of high-precision real-time three-dimensional measurement of large-size free-form surface workpieces, a three-dimensional measurement system for an ultra-large scale line-structured light sensor is designed and an onsite calibration approach for the extrinsic parameters of this sensor based on one-dimensional targets is proposed. The experimental results show that the proposed calibration method has a equivalent accuracy to compare with that of the planar-target-based method. Moreover, the proposed calibration method is easier for the accurate fabrication of the one-dimensional targets, more convenient for the onsite calibration process of the extrinsic parameters, and more suitable for the calibration of ultra-large scale line-structured light sensor.