Machine tools play an important role in the manufacturing industry. The straightness error of the guide rail during machining seriously affects the accuracy of the machine tool, which is usually measured based on the laser collimation principle by the quadrant detector (QD). Laser beam radius varies significantly with long propagation distances. However, the traditional measurement model hardly considers the impact of radius variation. Therefore, an improved straightness measurement model is proposed. The spot radius variation is taken into account in the improved model. Meanwhile, an accurate trajectory equation for the variation of beam radius with distance was quickly established using less measured data. It facilitates faster and more accurate straightness measurements in industrial sites. The feasibility of the improved model is verified at four distances during 5 m. At 3.5 m, the maximum calibration error of the improved model was −1.65 μm, which is 86.8% lower than the traditional model. Meanwhile, the root mean square error of the improved model was 0.7 μm, which was 83.1% lower than the traditional model. The maximum repeatability error of the straightness was 0.28 μm. The straightness measurement accuracy of the improved model is obviously improved at long distances. The improved model would have great potential for long-distance measurement using lasers based on QD.