To improve the straightness accuracy during the five-degree-of-freedom measurement （5DOFM） of long guide， the measurement light path， error model， and calibration method of the straightness measurement unit in the 5DOFM device were examined. During long distance measurement， the angle drift of the reference beam causes severe straightness error. Accordingly， a telescopic objective lens structure was adopted to measure the angle drift in the optical path. Compared with a single lens， the focal length can be increased in a limited space， such that the combined focal length is larger than the mechanical size， thereby improving the measurement accuracy， and compensating for the angle drift to improve the measurement stability. Additionally， the error introduced by angle crosstalk was analyzed， and a compensation model was established based on the angle measurement value in the device. A four-quadrant detector was used as the straightness measurement sensor. To solve the problems that the nonlinearity of the sensor increases as the range increases， and the sensitivity changes nonlinearly with the movement of the long guide， an interval calibration method combining polynomial fitting and cubic spline interpolation fitting was proposed. The experimental results showed that within the 5-m measurement distance and ±400-μm range， the error standard deviation was less than 0.3 μm compared with the measurement results of the laser interferometer， and the drift was less than ±0.5 μm within 3 h. Thus， the straightness measurement unit meets the requirements of large range， high precision， and high stability of long guide straightness measurement.