Accurate measurement of length is an important foundation for ensuring the quality of advanced manufacturing equipment. In recent years, absolute ranging technology represented by frequency scanning interferometry (FSI) has gradually become a widely used ranging method in the manufacturing industry due to its advantages of high precision, high flexibility, and no range ambiguity. To address the repeatability and accuracy of length reference calibration in FSI absolute ranging, this paper proposes a method of accurately calibrating length reference based on bidirectional absorption spectrum feature fusion and proportional coefficient calibration, by using gas absorption spectroscopy to calibrate the delayed long fiber path length as a length reference in the distance measurement system online, and by using weighted linear least squares method to solve the differences in uncertainty among different absorption spectrum peaks. To address the problem of low repeatability in optical fiber path length calibration by using absorption spectroscopy, a method of utilizing bidirectional absorption spectrum feature fusion is proposed, thereby improving the precision of optical fiber path length calibration. To address the issue of insufficient accuracy in absolute optical frequency of absorption spectrum peaks, a calibration method by using a single absorption spectrum proportional coefficient is proposed. Compared with the idea of calibrating the optical frequency of each peak one by one, this method is simple and direct, thus improving the accuracy of fiber path length calibration. To verify the effectiveness of the above methods, the experiments on repeated precision evaluation, proportional coefficient calibration, and accuracy comparison are conducted separately. The experimental results show that the standard deviation for calibrating the optical path length of 164 m fiber is 10–30 μm. Under the conditions of system temperature rise and temperature stability, the distance measurement standard deviations are not greater than 5 μm in the measurement ranges of 0–10 m and 0–15 m, and the distance comparison residuals are not greater than ±4 μm, demonstrating the good distance measurement performance of the system. In the future, we will carry out thermal insulation and temperature control of the gas absorption chamber and the entire ranging optical path, and study the stability of the spectral proportionality coefficient and absorption peaks while controlling external environmental factors.