To meet the precision requirements for full-stroke, multi-parameter composite measurement of rolling linear guides in ultra-precision machining, this paper investigates a geometric error compensation method for guide rail contours based on fused line-structured light sensing technology. To address the distortion in measurement data caused by the coupling of feed-axis straightness errors and sensor installation deviations, a unified mathematical model incorporating three-dimensional position error, attitude error, and sensor three-axis installation deviation is established based on multi-body system theory. The feed axis is treated as the "moving body" and the sensor as the "end effector," and the nonlinear coupling mechanism of various errors is clarified. An error identification method using a quadrant detector and a linear laser sensor is designed. Feed-axis position and attitude errors are separated through quadratic polynomial fitting, and sensor installation deviation angles are extracted based on geometric projection relationships. A joint compensation matrix is then constructed to correct and compensate the measurement coordinate system. An experimental platform for the guide rail geometric contour instrument is established. Straightness error identification is verified over an 800 mm stroke, and sensor installation deviation is validated within a 0.5°~2.0° deflection range. Identification errors for both methods are below 3.5%, confirming the accuracy of the proposed method for guide rail contour measurement. Comparative experiments under no compensation, single-error compensation, and joint compensation scenarios demonstrate that, after joint compensation, the average deviation of guide rail profile measurement decreases from 6.229 μm to 2.301 μm, with the mean deviation reduced by 36.9%, the standard deviation by 22.8%, and the maximum deviation by 20.8%. Full-stroke measurement accuracy is significantly improved, verifying the effectiveness of the multi-body system error model and the joint compensation matrix. The proposed method provides a theoretical and technical foundation for high-precision inspection of ultra-precision machining equipment.