3D object cut length measurement using light-knife is deeply researched based on laser triangulation,and the measurement principle and calibration method are studied in detail. The object-image relationships in height direction and light-knife direction are analyzed through coordinate transformation method,and the calculation formula of 3D object cut length measurement using light-knife is derived. A new linear calibration method is proposed based on the object-image relationships. An experiment of finger’s length measurement has been done and the results are precise. In the height direction the inverse of object’s relative height is linear with the that of image’s relative height. In the light-knife direction the ratio of object’s length to image’s length is linear with the object’s relative height. The new linear calibration method is very precise and simple. The advantages of 3D object cut length measurement using light-knife are of high accuracy,low cost,and high processing speed.