A self-collimating optical measurement system was set up to measure the deflection angles of three-dimensional objects, in which the values of the deflection angles were obtained by extracting the coordinate of a turntable cross wire. First, Sobel operator was used to detect the image edge of the cross wire taken by a CCD camera, then lines of the cross wire were extracted through self-adaptive threshold division. Because of other turntable scratches, the partial least square method was used to extract the cross wire, and the weighted total least squares was adapted to fit the cross-wire lines. Thus two equations were set up, and the intersection coordinate was obtained through solving the two equations. Experimental results show that the precision of the two line-slope product is within ±1%, very close to the true value (theoretical true value is -1). By taking the angle from Leica theodolite (the accuracy is 0.5″) as the true value to calibrate the precision, the measured precision angles for α and β are 3.59″ and 3.76″, respectively, which satisfies the requirements of deflection angle measurement.